Planning: Curb Cuts and Alleys

NB: I believe this topic has been discussed much better many times elsewhere on the net and in print.  I am writing simply to clarify my own thoughts.  I apologize for the primitive graphics.

One of the issues that arises with higher-density housing is parking.  As I discussed in a previous post, people want cars and thus parking for a variety of reasons.  On-street parking in residential areas has a number of issues, and off-street parking does as well.  Nonetheless, people desire parking, and there are ways to accommodate it in the context of fee-simple houses - up to a certain point.

The standard way to accommodate parking is to build access from the street to the private property where an owner wants to park vehicles.  In low-density suburbs and rural areas, that means a simple paved or unpaved driveway leading into the property from the roadway.  As housing becomes denser, in most cases the access crosses the pedestrian portion of the right-of-way, which is usually a paved sidewalk, but may in some cases be bicycle lanes.  These intersecting paths of movement are point of conflict between vehicles and pedestrians.  They are also known as curb cuts in America, because the standard high curb is cut down so the vehicle can drive smoothly from the street into a private property.  

On lots with large, open yards, it can be easy for a driver to spot a pedestrian.  (In this post, I will use the word 'pedestrian' as a shorthand for any using the sidewalk.  Apart from people walking, other users may include joggers, younger cyclists, micromobility users, assisted mobility users, and older cyclists, depending on the local regulations and specific configuration of the right-of-way.)  On lots where vision is obstructed by plants, fences, or other buildings, pedestrians can be injured if a driver is not careful.  Pedestrians learn to be wary in low-visibility situations, but too many dangerous crossings can make walking down certain streets unpleasant, and those streets are avoided.

Another issue is that curbside portion of a street where the access to private property ends becomes an area that can't be used for parking if there is on-street parking on the same side of the street.  In that situation, it can't be used for normal vehicular movement, either.  In some neighborhoods, an informal rule develops that a property owner can block the access to their own property, but doing so still risks a ticket.  It can also encourage less conscientious drivers to block other people's driveways.

A third issue with curb cuts is that sometimes people simply park their vehicles across the sidewalk, forcing pedestrians to detour and potentially enter the roadway to continue their journey.  This happens most often when the driveway is relatively short and a larger vehicle such as a pickup does not fit completely onto the private lot.  It also happens when the driveway is obviously too short, but the vehicle fits between the end of the driveway (usually a garage door) and the street.  Either way, in most jurisdictions this discourtesy is illegal, but rarely enforced unless someone complains.

One solution to the issue of curb cuts is to create a second, usually parallel, right-of-way that allows rear access to properties for the purpose of parking.  This right-of-way is usually called an alley, but is sometimes called a service lane, or just a lane.  Alleys predate the automobile by hundreds or thousands of years, depending on how they are defined. Some American cities that have them were laid out when the horse was the main means of long-distance travel.  But alleys have been adapted to support motor vehicles, and offer some advantages over front access for parking.

The first is that alleys cut down on the opportunities for pedestrian-vehicle conflict.  Alleys reduce the number of curb cuts from many to as few as one per block, though two is most common.  While pedestrians can use the alley as a walkway, most people prefer a sidewalk, as an alley is an obvious zone of conflict.  Some urbanists are interested in upgrading the alley to a sort of second, pedestrian friendly street, but outside of warm tourist towns, that seems to me to be a duplicative and futile effort.

The second advantage is that with rear parking and reduction of curb cuts, the street in front of homes has more space either for parking or the various forms of traffic, depending on how it is striped.  Or the roadway can be narrowed and restricted to vehicle movement only, reducing the amount of paved area the jurisdiction needs to maintain.  The space freed in this way could then be used to create or expand curb lawns.

Alleys have some smaller benefits.  They are good places for trash pickup, as a garbage or recycling truck can access cans or bins in an alley without having to pull them across a line of parked cars from the sidewalk or curb lawn.  They also reduce the chances of the truck driver coming into conflict with moving vehicles, as alleys are usually so narrow that most passenger vehicles cannot pass a stopped truck.  They can also be used for overhead utilities in areas where they cannot be placed underground due to costs or technical reasons.  Overhead wires in an alley don't mar the appearance of the adjacent street, and they don't come into conflict with any trees planted in front of the homes.

Alleys have downsides, of course.  The primary one is that they have to be built and maintained.  While they don't have many of the features of streets, such as sidewalks, curbs, crosswalks, lane markings, and traffic lights, they still need to be built to support the vehicles that travel down it.  Garbage trucks are the heaviest vehicles to travel through residential areas on a regular basis, and cause the most damage to residential streets from vehicles.  The second is that the hard surface of an alley will increase runoff over that of unimproved yards.  The tradeoff between having an alley and rear parking verses having driveways and front-parking needs to be calculated when it comes to managing storm water.

Another negative negative factor is that alleys tend to be unsightly.  Because they may not be fully visible from the surrounding buildings, trash can be dumped and improvements vandalized without anyone even being able to notice.  And the fences and structures along alleys are often neglected since they aren't the public face of the property.  Alleys are problematic in areas with heavy snowfall.  Because there is nowhere to put the snow, they are left unplowed, making access difficult or dangerous.  Snow can be moved with a bucket-loader and trucked out, but that is an expensive option.

Alleys also have a reputational issue.  The dark alley is a widespread fictional device that probably has its roots in the actual alleys of medieval cities in England.  There is no doubt that the originals were poorly lit and often the location of crimes that might not take place in wider streets or the daylight.  Modern alleys are not narrow walkways with sections passing beneath buildings, and often have streetlights.  Nonetheless, the negative perception continues, probably because alleys tend to be trash-strewn and poorly maintained, indicating that few people notice or care what happens in them.  Research indicates that gating alleys in the UK reduces crime by a small amount, but the number of studies is limited.  The overall level of crime in an area with alleys is probably more important than the absence or presence of alleys themselves.

Because of their complications, I think alleys need to be analyzed with caution in mind before they are included in a design.  I think they provide a way to mitigate the issues of parking in residential areas of moderate density, but they are not a solution in themselves.  Only by addressing car-dependent mobility can the problem of parking be truly solved.

Planning: Block Sizes for Mixed Uses

NB: I believe this topic has been discussed much better many times elsewhere on the net and in print.  I am writing simply to clarify my own thoughts.  I apologize for the primitive graphics.

In a previous post I examined block sizes for perimeter apartments, a common form of housing in Europe.  They allow fairly high density, but generally don't allow for very large businesses to be established on their ground floors.  Many businesses don't necessarily need or even want street-level access, and could occupy higher floors.  Others want much larger floor plates, or need to be in a separate structure for reasons of noise, vibration, or other measures.  Replacing the portion of the perimeter apartments with a building dedicated to businesses is one way to integrate the two functions.

Here I have depicted some fairly simple divisions of a block into thinner perimeter apartments and commercial buildings that use up their entire lots.  The main issue is the amount of open space from any window to a wall or window on a facing building.  This is culturally dependent, and ranges from a few feet to as high as 80 feet, even in urban areas.  New York City has codified a minimum of 30 feet from a window to a wall, and 40 feet from a window to a window, but only up to 25' in building height.  Offset distances increase with building height.   For a building 55' high, as I have been using, the values would be 50' and 60', meaning the commercial building in the upper left corner could not be inserted into the block without carving it back to the T-shaped area outlined on the roof.  However, if it was built first to the limits of the lot, then subsequent apartments might have to be impractically thin.  The zoning code may have rules for resolving, preventing, or mitigating conflicts of this kind, or it may depend on negotiation and trading of rights.  No matter what the method, the conflicts would need to be resolved clearly.  Otherwise, the first landowner to build in any block could seriously impact the use of every other owner on the block without the others' consent or foreknowledge.  There would be less conflicts on blocks larger than the 260' by 260' (buildable) examples I have shown above.

For larger businesses, it may be necessary to develop entire block at one time, and replace the entire interior of the block with the commercial structure.  This is often done with parking garages in suburban developments, and results in what has been dubbed the Texas doughnut.  This configuration has the advantage of hiding pedestrian-unfriendly or generally unappealing buildings away from sight.  Structures hidden in this manner can cut some costs since aesthetics are not a concern.  However, the arrangement could reduce the exterior ring of apartments to outward-facing units only, meaning that common elements are shared between fewer units, increasing per unit costs.  The density of the block would be less than with double-sided blocks, but the tradeoff may be worthwhile in some circumstances.  With bigger blocks, it would be easier to develop a large commercial structure with little impact on adjacent buildings.

Another approach available only to large development projects is to stack the functions.   By putting the business functions on the outside and constructing a residential tower above that on the inside, businesses could have fairly large floors.  However, the apartments could possibly look out on an unattractive roofscape full of machinery, solar panels (hopefully), or highly reflective roofing material, though "green roofs" could be an option.  They would also lose any connection to the street below, giving them a suburban feel at best.  Inverting those functions would work better in some ways, with the apartments looking directly onto the street, and the businesses looking onto a relatively small roof area, along with the street, at least from higher floors.  However, the major disadvantage here would be a large area of interior space below the tower that would have limited uses, such as storage or parking.  Parking located here would be fairly expensive, as it would all be custom-designed and cast-in-place in order to fit inside the envelope and have a column grid compatible with the businesses above.  More complicated architectural forms could potentially make the large interior area less useful, though they would benefit non-occupants if they had lasting aesthetic appeal.  Overall, I think smaller blocks would work better for whole-block developments, but it would depend a lot on local expectations and regulations related to heights and setbacks. 

In addition to businesses of various sorts, there are a number of other institutions that are part of any urban fabric, including religious organizations, schools, universities, hospitals, government facilities, and museums.  All of those will have a physical presence.  Often they are large enough to require an entire block, or are given one in order to highlight their prestige.  But smaller institutional buildings can be easily integrated into a block of apartments.

Investigating the mixture of apartments with larger commercial or civic functions leaves less clear answers about block sizes, because the non-residential functions have widely varying needs.  In most dense areas, I feel the established grid should force the commercial or civic function to adapt.  There may be a few situations where institutions need to combine blocks to function effectively, such as sports arenas or conference centers.  But those should be rare exceptions, and should be sited so the superblocks don't interrupt important traffic corridors or divide neighborhoods.   In a future post, I will look at block sizes for larger businesses.

Planning: Block Sizes for High-Density Housing

NB: I believe this topic has been discussed much better many times elsewhere on the net and in print.  I am writing simply to clarify my own thoughts.  I apologize for the primitive graphics.

In a previous post I examined block sizes for perimeter apartments, an arrangement found in Europe that can create high-density neighborhoods.   In this post, I will look at other high-density apartment building types, mostly from New York City.

NYC has been America's most populous city since the first census in 1790, and contains one of the largest extents of hyperdensity on the planet.  Due to the restrictions imposed by the surrounding waterways, land has long been valuable in the city, leading to intensive development.  When the majority of the Manhattan was laid out in 1811, the population was only about 100,000.  In contrast, London had a population of about 1,200,000, and Paris about 600,000.  The blocks in the undeveloped parts of NYC were divided mostly into lots measuring 25' by 100'.  This was deemed appropriate for a single-family house at the time, whether attached or detached.  However, as the city grew rapidly to 810,000 by 1860, the lot width became a liability, because the single family houses were replaced with tenements, or apartments, for low-income residents, which were the majority of the population.  The earliest of these ("pre-law") were often built to the limits of the lot, with only a 10' deep rear yard.  This meant that interior rooms received no light or fresh air, as light shafts or courtyards would cut into profits of the landlords

This led to the "First" Tenement House Act of 1867, which required fire escapes for each apartment and windows for each room, among several requirements.  Landlords met the letter of the law by installing windows in interior walls, which may have provided a slight improvement for some rooms, but was not what the legislators had intended.  In 1879 the "Second" Tenement House Act was passed, which required windows to face the street, a yard, or a minimal airshaft open to the sky.  This airshaft was very small by design, to please developers, and did little to light rooms.  Unfortunately, the narrow air shaft led to two problems.  One was that it was used as a dumping area for trash and waste by residents on upper floors, leading to unpleasant odors for all because the space was not designed to be easily accessed for cleaning.  The second was that it enabled fires to spread floor-to-floor and building-to-building more easily, as the air shaft acted as a flue.

The problems with the "Old Law" led to the "New Law" formally known as the New York State Tenement House Act, passed in 1901.  This new legislation required "inner courts" entirely enclosed by the property to be at least 24' by 24' for a 60' tall building.  For courts on lot lines, the minimum was 12' by 24', which could be paired with the adjacent building for a more open space.  The law also allowed "outer courts" to extend from deep inside the building to a street or back yard.  These had an minimum dimension of 6' when on a lot line, and 12' when between parts of the same building.  The measurements again were for a 60' tall building.  The regulations for both inner and outer courts had adjustments for both taller and shorter buildings, as well as absolute minimums.

Above I have depicted the basic shapes allowed in each era.  First, at the bottom, are the "pre law" tenements that were allowed up to 1879, with no air shafts at all.  Next are the "old law" "dumbbell" tenements, with the problematic narrow air shafts.  On the upper block are examples of inner court and outer court "new law" buildings.  Multi-family dwellings on lots under 40' or so became much more difficult to build economically after passage of the law, thus the wider buildings.  Pre law and old law tenements were mostly 4 or 5 stories in height, but some new law tenements were as much as 6 or 7 stories, even without elevators, in order to make up for the lot space reserved by the new requirements.  Similarly-sized buildings conforming to the 1901 statute were also constructed for more upscale customers, with larger suites and at least one elevator.

Another major city experiencing explosive growth about the same time was Berlin.  Previously the capital of Prussia, it was made the capital of a united Germany in 1871.  The population then grew from 826,000 to 1.88 million by 1900, roughly the same as Manhattan at the time.  It also had tenements, known as mietskaserne, or "rental barracks."  These buildings may have started as perimeter blocks, with additional structures added first at the rear of each, and then along the sides, to create large interior courts.  But by the late 1800s, they were regularly built to the lot line from the start.  Unlike in NY, there were not separate buildings for the wealthy and the poor.  Instead, the wealthy lived in the front section on the second floor, while others crowded into the rest of the building.  Blocks in Berlin are much less regular than in Manhattan, and the image above shows an abstraction of the basic form.  The mietskaserne were often extended to create a second or even third courtyard on deeper blocks.  This did not happen to tenements in New York, because the more rigid street layout created few deep blocks.  The meitskaserne could also be reoriented to pair with units on the opposite side of a very shallow block  Also included in the image for comparison is a block with identical dimensions and perimeter apartments. 

	Perimeter, 60' thick	25' Pre-law	25' Dumbell	50' New Law I-plan	50' Mietskaserne
Gross length	900	900	900	900	900
Gross depth	260	260	260	260	260
ROW width	60/100	60/100	60/100	60/100	60/100
Net length	800	800	800	800	800
Net depth	200	200	200	200	200
Lot width	580	25	25	50	50
Lot depth	130	100	100	100	100
Lots per block	2	64	64	32	32
Units per floor	100	4	4	6	7
Floors	5	5	5	5	5
Lot coverage	66.00%	90.00%	78.00%	68.00%	72.16%
ROW per unit (ft.)	2.32	1.81	1.81	2.42	2.07
Density per block (pers./sq. mi.)	119,138	152,497	152,497	114,373	133,435

Because of their very high lot coverage ratios, the pre-law and dumbell tenements generate the highest density.  New law buildings cover less of their lot, which accounts for the lower density.  However, they are by all accounts much more pleasant to live in.  An argument can be made that the mietskaserne are better still, because they have one large courtyard instead of a smaller courtyard and a rear yard that isn't very usable.  They are the only type still built as well, as the NYC zoning code has continued to be updated and nothing in the shape of the new law tenements is built.  Block sizes for the mietskaserne should probably be driven by a balance of walkability and efficiency, similar to perimeter apartments.  The NYC blocks are too long, so something around the size I have previously been using (580' by 240' buildable) would preferable.  However, some blocks with mietskaserne in Berlin are as large as 700' by 800', with over 35 interior courts.  That level of interaction between different structures might be a difficult to sustain in societies where businesses relationships tend to be more adversarial.

Much larger buildings were also constructed within the regulations of the 1901 law.  Show below are three whole block buildings.  From smaller to larger they are The Apthorp (1908), The Belnord (1909), and London Terrace (1931), which is actually 14 adjacent buildings.  The first two are luxury buildings with large units and separate servants' elevators and entrances.  The last was designed as a middle-class housing for workers in Midtown Manhattan.  Its units ranged from studios to two bedroom units, with some larger penthouse units.  Originally housing over 4000 residents in 1665 units, it remains one of the largest apartment buildings in the world.

Filling a lot right up to the edge of the surrounding right of way is not the only way to create density.  Isolated towers can be constructed with enough floors to substitute for high lot coverage.  In fact, that was the point of one of the earliest proponents of the "tower in the park" arrangement, Charles-Edouard Jeanneret.  However, in reality most residential towers aren't surrounded by parkland, but parking garages, indoor malls, or other pedestrian-unfriendly features.  In some cities, most notably Vancouver, the towers are located in or on a podium of human-scaled shops and housing, but that arrangement is not the rule.  Isolated towers in NYC, whether privately or publicly-owned, are usually not surrounded by other buildings.  But neither do they float in a sea of undisturbed nature; land there is too valuable to allow that.  In most cases the spaces surrounding NYC towers are simply small sections of fenced-off grass, with a few trees and not a lot of other landscaping.

	Perimeter, 60' thick	Belnord	London Terrace	UdH-style towers	HK-style towers
Gross length	900	440	900	900	900
Gross depth	260	260	260	260	260
ROW width	60/100	60/100	60/100	60/100	60/100
Net length	800	340	800	800	800
Net depth	200	200	200	200	200
Lot width	800	340	800	800	800
Lot depth	130	200	200	200	200
Lots per block	2	1	1	1	6
Units per floor	100	18	100	40	8
Floors	5	12	16	23	36
Lot coverage	66.00%	68.61%	60.40%	25.50%	21.14%
ROW per unit (ft.)	2.32	6.48	1.45	2.52	1.34
Density per block (pers./sq. mi.)	119,138	115,803	228,746	109,607	205,871

There are a two things to note in the table above.  First, the densities of both The Belnord and the London Terrace complex are based on census data, instead of being calculated.  Because they are expensive properties, they may have a higher number of small households and second homes than the rest of the city, so the density shown is probably lower than I would calculate.  Unfortunately, I don't have access to current floor plans, so calculation is impossible anyway.  Second, the block sizes for the two tower-style buildings are arbitrarily the same size as the others, which affects the density calculations.  In reality, because they are intended to have little relation ship with the streets below, block sizes don't matter that much.  The vast majority of similar towers have been built in China, Hong Kong, Singapore, and Korea, where they are part of large planned unit developments (PUD), and have a very suburban feel.  That is unlike The Belnord and London Terrace, which arguably may overwhelm their adjacent streets, but don't entirely turn their back on them.

Ideal block sizes for the isolated tower blocks are essentially impossible to determine, because of their lack of relationship to surrounding streets.  Preferably, they would not be built in an urban environment, but in NYC many have, often arranged in geometrical patterns on superblocks that obliterated existing streets.  For the whole block-buildings, a shorter block works better because it promotes walkability.  London Terrace should have been be split between two different blocks, and have more retail or office space on the ground floor, but NYC does not have a history of splitting large blocks to this day.  The depth of the block for the three probably depends on what people feel about the tradeoff between density and light.  North-facing exposures on lower floors will never get much natural light, but somewhat deeper blocks would increase the amount.  The tradeoff would be lower density and higher land costs per unit.

At this point I have explored most housing types that are built in high and middle-income countries.  In a future post I will look at some commercial structures and whether then can be constructed within the confines of a urban grid.

Planning: Block Sizes for Small Multi-Unit Buildings

NB: I believe this topic has been discussed much better many times elsewhere on the net and in print.  I am writing simply to clarify my own thoughts.  I apologize for the primitive graphics.

In previous posts on block sizes, I have looked at lots with a single dwelling unit, which is the most common form of home in America.  But to build an area to a density higher than that of row houses, multiple units need to be develop on the same property.  That means other forms of both buildings and ownership need to be investigated.

In America, there are functionally four types of home ownership:  fee simple, homeowner association, condominium, and cooperative.  These have been explained in great detail elsewhere, so I have provided links since I have nothing further to add.  However, I will note that while single-family detached homes can be built under all four types, individual ownership in multi-unit buildings are always of the later two.  Of course, a multi-unit building can be constructed by a single owner for the purpose of permanently renting the units, and many are.  Public housing is a subset of that arrangement.

The easiest way to create multiple dwelling units on one lot is to simply subdivide an existing single-family home.  This is often done - whether or not it is allowed by local regulations - to older homes that were built when families were larger.  New homes with fewer bedrooms are also partitioned if the basement or existing in-law suite can be isolated from the main portion of the house.  The additional income from the rental unit can be important to addition to a household's finances, especially for retired homeowners.

Another way to create multiple dwelling units on a lot is to create a second structure at the rear, usually along an alley, but sometimes accessed from the front along a common driveway.  The formal name applied by many American jurisdictions is accessory dwelling unit, or ADU.  The units are almost always smaller than the main home, containing 0 (a studio), 1, or 2 bedrooms.  They are often located above a garage for the main dwelling, and are usually owned by the same person who owns and occupies the main unit.  Some jurisdictions prohibit ADUs from being subdivided into a separate property, requiring them to be rented or used as guest houses in perpetuity.

While the above actions add diversity to existing neighborhoods, existing smaller units are not a significant portion of any nation's housing stock.  The main way multi-unit dwellings are created is to design them as such from the beginning.  Purpose-built apartments range from simple up-down duplexes found in cities of the Northeast, through mid-rise one-plus-five buildings being developed in suburban areas across America, to super-tall luxury skyscrapers in major cities like New York and Hong Kong.   They can be built to be rented, to be sold as condominiums, or to be incorporated as cooperatives, though the latter is unusual in the 21st century.

Because subdividing existing homes happens after a neighborhood is laid out, there's no reason to investigate the impact of such actions on the layout of blocks.  However, a neighborhood can be designed to allow the next step up in making multiple units, which is allowing for or requiring ADUs, and they should be examined.

For side-by-side duplexes or single family homes, there appears to be no reason to change the size of the block.  The addition of a second story to the garage primarily changes the depth of the shade in the back yard and in the alley.  Depending on the latitude and local climate, that may make the yard a little dark and damp at times, but could also act to cool the space in other areas.  Most ADUs allowed by law in America would not change the size of the yard significantly over having a simple garage. 

For row houses, an ADU has similar effects on the yard.  But on a 24 foot wide lot, the stairs from the second floor to the alley will impinge on the garage area.  Because of the large cars prevalent in America, the number of indoor parking spaces would be reduced to one, unless the owners purposely bought much smaller vehicles than is typical.  Most owners would probably just use the inadequate spot for storage and continue using a larger vehicle.  It is also be possible to build an ADU on 20' and 16' lots with row houses, though again the stairs would restrict the amount of parking and storage space.  But nothing necessitates a change to the overall block size.

	32' DFH	32' DFH w/ rear ADU	24' SDFH	24' SDFH w/ rear ADU	24' AFH w/ rear ADU
Gross length	640	640	640	640	640
Gross depth	320	320	320	320	320
ROW width	60	60	60	60	60
Net length	580	580	580	580	580
Net depth	260	260	260	260	260
Lot width	40	40	30	30	24
Lot depth	120	120	120	120	120
Lots per block	30	30	38	38	48
Parking – private (spots)	2	2	2	2	1
Parking – street (spots)	1	1	0.75	0.75	0.6
Driveway or alley (sq. ft.)	400	400	300	300	240
Pct. paved	7.69%	7.69%	7.69%	7.69%	7.69%
Back yard (sq. ft.)	1680	1680	1260	1260	1008
Pct. back yard	32.31%	32.31%	32.31%	32.31%	32.31%
ROW per unit (ft.)	64.0	64.0	50.5	50.5	40.0
Density per block (per/sq. mi.)	16,335	20,419	20,691	25,864	32,670

Unsurprisingly, building an above-garage ADU does not increase density radically.  In fact, using my assumptions of 1 person per small bedroom, and 2 per large bedroom, a 0 or 1 bedroom ADU would increase density by about 25%.  That is a significant increase, but the absolute is still fairly low compared to many urban jurisdictions.

Changing the planning paradigm to taller dedicated apartment buildings is the way cities like New York, Paris, and Hong Kong achieve high density.  However, not every dedicated multi-unit property is a skyscraper.  In fact, there is more diversity of types of multi-unit properties than there are of fee-simple homes, which amount to two - detached, and party wall (duplex or row).  Many multi-unit properties are constructed in large developments in very suburban areas, in an arrangements with low overall density.  But others can be worked into the fabric of older neighborhoods. 

Here I have depicted a mix of single-family homes, duplexes, and what are often called sixplexes, or three-floor, six-unit apartment buildings.  The multi-unit buildings are larger, but don't overwhelm the surrounding buildings.  The same basic lot layout is preserved, with the dwelling structures close to the street, and parking structures at the rear.  Usually a multi-unit building in this context would not have garages, however, and would instead simply have a lot, as an open area can accommodate more cars and costs significantly less.

Other "plexes" exist, such as triplexes (three units stacked) and fourplexes (two units wide by two high).  More complicated arrangements are possible while keeping the building size reasonable, but builders frequently stack and mirror units to reduce costs.

	24' SDFH w/ rear ADU	32' Triplex, rear garage	40' Four-plex, rear garage	40' Six-plex, rear garage	30' 3-2BR APT, attached
Gross length	640	640	640	640	640
Gross depth	320	320	320	320	320
ROW width	60	60	60	60	60
Net length	580	580	580	580	580
Net depth	260	260	260	260	260
Lot width	30	48	58	58	30
Lot depth	120	120	120	120	120
Lots per block	38	24	20	20	38
Parking – private (spots)	2	4	5	5	2
Parking – street (spots)	0.75	1.25	1.5	1.5	0.75
Driveway or alley (sq. ft.)	300	480	580	580	300
Pct. paved	7.69%	7.69%	7.69%	7.69%	7.69%
Back yard (sq. ft.)	1260	1248	1508	1508	1260
Pct. back yard	32.31%	20.00%	20.00%	20.00%	32.31%
ROW per unit (ft.)	50.5	80.0	96.0	96.0	50.5
Density per block (per/sq. mi.)	25,864	39,204	43,560	65,340	46,555

The three different plexes in the table each reduce the back yard area significantly, but that's not really a major issue, as a common back yard is rarely used.  The better way to provide private outdoor space for upper floor apartments is to add a balcony.  A similar area can be sectioned off for ground-floor units in most circumstances.  The most important thing to note is that a neighborhood of sixplex buildings can result in fairly high density - even higher than narrow attached buildings of three stacked two bedroom units.  The difference is mainly due to the increased number of bedrooms per unit.

Three floors is a critical cutoff point in America, above which accessibility requirements mandate elevators, and fire codes require either sprinklers or a change in construction materials, or both. In a future post I will look at some apartment building forms that are constructed to greater heights.

Planning: Block Sizes for Perimeter Apartments

NB: I believe this topic has been discussed much better many times elsewhere on the net and in print.  I am writing simply to clarify my own thoughts.  I apologize for the primitive graphics.

Over a number of posts I have discussed block sizes for progressively smaller blocks and denser housing arrangements.  In the last one on the topic, I looked at smaller multi-unit buildings, and found that they continue to fit well into the 260' deep by 580' long (buildable area) block I have been using in my images and calculations for several posts.  I this post, I will examine perimeter apartments, which is a common arrangement in European countries such as Sweden, Germany, and Czechia.

Perimeter apartments are arranged into perimeter blocks.  These are distinct from courtyard buildings, where a single building is constructed with some street-facing rooms, but also with a number of rooms that face an internal open-air space.  Individual homes can be designed to face a courtyard as well, and examples can be found at least as far back time as Pompeii before the eruption of Vesuvius.  With perimeter blocks, the courtyard is shared between several different buildings.  Blocks of row houses have a superficial similarity, but with that building type the internal space is clearly partitioned into private lots, with no common areas.

Note that I have not included lots in the following images.  That is because ownership seems to vary within and between countries, and language barriers prevent me from easily pursuing the issue.  However, what is important is the general form, and the agreement - whether through cultural norms or specific regulations - that the interior is not developed.  (At least initially - over time the central courtyard is often filled in.)

Both the horizontal depth, or thickness, of the surrounding buildings, and their height, results from the regulations and practices followed in each country.  In much of Europe, apartments have historically been constructed to 4 or 5 stories with a single staircase, which eliminates the need for interior corridors to separate the vertical circulation paths.  That allows buildings to be as thin as 36'-40'.  More typical in older areas is a thickness of 44'-48'.  When multiple stairways and corridors are required, they are typically placed inside, pushing the dimension to 56'-60'.  Higher figures are seen in luxury properties, where the rooms are much larger.  If an exterior corridor is used, buildings can be very thin, but that arrangement is generally unpopular, and confined mostly to isolated towers.

In the image above, the block size depicted is again 260' by 580' buildable.  Clockwise from the lower left, the building thicknesses are 40', 50', 60', and 70'.  The height is 55', or 5 stories with a half-exposed basement and parapet. With the thinner buildings, the interior courtyard is quite large, and the distance between the interior apartments on the long sides is almost three times that of exterior apartments to blocks across the right-of-way.  Occupants of interior apartments on the short sides would barely even notice the far end.  The interior apartments on the long sides of the thickest building are still separated by twice the width of the street.  Clearly blocks developed in the perimeter form could be reduced in size by a significant amount before the units are adversely impacted. 

Here I have simply reduced the blocks to half their length.  The courtyards are still larger than the street width, but the space is no longer sufficient to host an (American) football game.  Nonetheless, it would still be possible to put a playground, a basketball or tennis court, and a gazebo for picnics within the space - if residents desired such amenities.  These would be shared between fewer units, increasing the cost per unit, but would have less demand for usage.  A clear advantage with smaller blocks is that the streets become more walkable, with more paths between destinations and more street frontage for businesses.  The effect is greater with shorter blocks (blocks that tend towards being square) than narrower blocks (blocks that tend towards being long and thin).

There are some technical tradeoffs between the thicknesses.  Thin buildings dictate that any spaces created for businesses on the ground floor would be relatively small.  Many businesses feel they need certain dimensions to operate effectively, and avoid smaller units.  Thick buildings provide larger floor plates, but also mean inside corners become difficult to allocate usefully on residential levels, increasing the price of each unit.   Thick perimeters also mean that a large portion of each unit will receive little to no natural light, because the inner portions are used for kitchens, bathrooms, storage, and other support functions that are in small, closed rooms.  At least some of those rooms have to be positioned on an exterior wall in thinner buildings, trading floor plan efficiency for natural light and fresh air.

The perimeter can be broken to resolve some of those problems, though at the price of the continuous perimeter, and an urban feel.  And once the perimeter is broken, the desire for parking could overwhelm the desire for open space, and the interiors of the block could end up covered in pavement.  Parking within the block would also reintroduce the problems associated with curb cuts.  However, a less rigid adherence to building to the limits of the property lines in each block would allow for more architectural expression and a wider variety of living experiences.

Above I have depicted some blocks based on one of the best known grids in urban planning, the Eixample ("Expansion") district of Barcelona, Spain.  They are much larger than the blocks I have been examining, with unique, chamfered corners.  Those cuts were made because the planner believed that large, cumbersome steam tractors - an advanced technology at the time - would be important for moving goods and people around the city.  Unfortunately, with current transportation technology, the extra space at the intersections is usually dedicated to parking, dumpsters, or ill-defined pavement, degrading the quality of the space.   Regulations implemented in 1958 allow for very thick perimeter apartments, as well as significant height increase over what the planner originally envisioned.  Almost all of the blocks have been fully enclosed as well as filled in, eliminating a key component of the plan, which was garden space available for all residents to enjoy.

	24' RH, deep block	40' Sixplex, 3BR units	Perimeter, 40' thick	Perimeter, 50' thick	Perimeter, 60' thick
Gross length	640	640	640	640	640
Gross depth	320	320	320	320	320
ROW width	60	60	60	60	60
Net length	580	580	580	580	580
Net depth	260	260	260	260	260
Lot width	24	58	580	580	580
Lot depth	120	120	130	130	130
Lots per block	48	20	2	2	2
Parking – private (spots)	2	5	0	0	0
Parking – street (spots)	0.6	1.5	0.11	0.07	0.09
Driveway or alley (sq. ft.)	240	580	0	0	0
Pct. paved	7.69%	7.69%	0.00%	0.00%	0.00%
Back yard (sq. ft.)	1008	1508	42312	71744	59664
Pct. back yard	32.31%	20.00%	56.12%	44.18%	36.74%
ROW per unit (ft.)	40.0	16.0	7.4	8.0	5.6
Density per block (per/sq. mi.)	26,136	65,340	89,843	103,455	133,403

I have used an assumption of two persons per large bedroom and one person per small bedroom to calculate densities, except for zero and one bedroom units, where I allocate one person regardless of size.  The density values also reflect actual floor plans, which I have not shown here.  The 40' thick buildings are laid out with 4 units per floor, with two facing out to the street, and two facing in towards the courtyard.  This is similar to "garden" apartments found in America.  The 50' thick buildings have only two units per floor, with exposures on both sides.  This arrangement is common in Europe.  The 60' buildings are fairly standard interior corridor buildings, with units ranging from 0 to 5 bedrooms.  These are seen in many countries.

In general, I think that the blocks for this form of building could be reduced in size in one dimension or another from the 260' by 580' block I have been working with.  However, the size remains effective, allowing for high densities.  In an urban area with multiple different building forms, continuity with other parts of the grid is important.  Without a pressing need to reduce the blocks, I will continue to work with the size I have been using.  In a future post I will examine block sizes for high-density apartment buildings.

Planning: Curb Lawns and Trees

A curb lawn, one name for a strip of vegetation running along a roadway, is a frequent element of rights-of-ways in America and other countries that have similar suburbs.  There is a wide variety of names for the feature, as documented here.  While often maligned for being nothing more than a monoculture of parched crabgrass, they can be important elements of streetscapes when utilized properly.

The best use of curb lawns is for planting trees.  Trees have a number of benefits to the users of a right of way, and to the residents that live nearby.  One is that they can provide shade for pedestrians, making their journey more pleasant on warmer days.  They can also shade hard surfaces such as sidewalks, roadways, and buildings, reducing the urban heat island effect.  Another benefit is that they absorb pollutants and particulates, improving the air nearby.  Trees benefit surrounding areas by absorbing storm runoff, reducing the chances of flash flooding, and the amount of water that has to be treated in combined sewer systems.

Trees also have less immediate benefits.  Foremost is that most people find a street planted with trees to be aesthetically pleasing.  This is generally reflected in property values, though it should be noted that may be more of a correlation than a causation.  Trees are pleasing not only for the shade they provide, but because they help shape the space for pedestrians, by providing a canopy that encloses and a prospect through which people can see other people and objects moving in the distance.  Large boulevards in European cities with double rows of trees excel at providing both qualities.

However, curb lawns have benefits even when they are not used to plant trees.  The primary benefit is to provide a buffer space between pedestrians and moving vehicles.  Without space to mitigate the sound and air movement created by a passing vehicle, many pedestrians find walking along busy roads unpleasant.  Another is that they allow for various streetscape items to be installed so they do not constrict or block the sidewalk.  Permanent items include signage, lampposts, telephone poles, fire hydrants, and traffic signals.  Other items may include bike racks, trash and recycling receptacles, restaurant seating, and benches.  These useful features add texture to the street and should be enabled as much as possible.

In narrower rights-of-way , where there isn't sufficient area for a continuous curb lawn, space for trees can be designed to alternate with space for parking.  Such arrangements still provide for both a buffer between pedestrians and moving vehicles, and most of the benefits of trees.  The compromise reduces the area for other vegetation and for the other minor uses that people find for the area, but are good nonetheless.  Busy roads with a line of bollards or similar devices to separate traffic from pedestrians may still be safe, but can appear unfriendly.

Some of the utility of curb lawns is dependent on climate.  In drier areas of the world, there may not be enough moisture for any vegetation to grow naturally in a curb lawn, let alone leafy, shade-providing trees.  Irrigation can be installed, but apart from high-traffic commercial areas, investing in such technology does not make sense.  It is reasonable in most arid regions to eliminate curb lawns along residential streets and place the sidewalk directly along the roadway, with the two areas only separated by a curb.  However, the important of a buffer space between pedestrians and vehicles should not be forgotten, and busier roads should still provide separation between the two groups of users.  The distance can be reduced with a physical barrier of sufficient height, but those are expensive and are mostly used along limited-access highways.

Ultimately, the presence of trees and curb lawns is a cultural practice.  There are many successful urban environments where neither are present.  However, there are good reasons for them to be installed where rainfall is sufficient.  They should not be discarded in the pursuit of either maximum density or maximum parking.

Planning: Block Sizes for Small Lots

NB: I believe this topic has been discussed much better many times elsewhere on the net and in print.  I am writing simply to clarify my own thoughts.  I apologize for the primitive graphics.

In a previous post I discussed block sizes for lots that are small enough that single-family detached houses - the kind of dwellings found in most suburbs and many older neighborhoods - stop making sense.  That happens at about 30' in lot width.  Below that figure, attached houses become the rational choice because side setbacks become too small to be anything other than a walkway, while at the same time reducing the width of the building so much that floor plans become inefficient.  Some jurisdictions allow detached houses to be built with essentially zero setbacks on two sides, but that is unusual.  In most locations, houses on narrow lots are attached on both sides, and are called row houses or town houses depending on their context.

Row houses can be built with front garages, rear garages accessed from alleys, private parking lots (almost always in suburban planned communities), or no off-street parking at all.  Front garages in row houses have the same problems as in narrow detached homes - unpleasant appearance, pedestrian-vehicle conflicts, and curb cuts.  In higher density areas, having rear garages or not having off-street parking at all are much better options.

Here I have depicted 24' wide row houses with rear garages.  The overall floor area is a little smaller than with the detached and duplex homes previously discussed, which is not unusual for urban areas where space is at a premium.  The back yard has shrunk significantly due to the reduction in width.  While still spacious by the standards of many countries, in America it would probably be considered by many to be too small.  Increasing the block depth would alleviate that problem somewhat.

When the block depth is changed, the block width should be re-examined.  Three blocks of 300' by 900' make a square that works at lower densities, but at higher densities, the degree to which longer blocks impair walkability becomes important.  Also, while there is no rule that requires blocks align when their orientation is changed, in the abstract doing so makes sense.  Shifting the block size to 320' by 640' would add 10' to each back yard while increasing the walkability, and still forming a square.  However, doing so would lower taxability due to the additional roadway area.  The alley present in this depiction also adds infrastructure that would need to be supported, though at a lower standard.

For American expectations, a 16' wide lot is about the smallest that can be created.  In other countries that have row-type homes, such as The Netherlands, lot widths of 14' or even 12' can be found.  Reducing the lot to 16' would actually decrease density over a 20' wide lot - provided the building remains only 2 stories high - due to the dimensions of the interior space.  Given American expectations and building technologies, only 2 bedrooms can be built in a 16' wide envelope - without resorting to very narrow exposures, which have fallen out of favor since the early 1900s.  Naturally, building higher would allow for more bedrooms.

	24' Duplex	24' Row House	24' RH, deep block	20' RH, deep block	16' RH, deep block
Gross length	900	900	640	640	640
Gross depth	300	300	320	320	320
ROW width	60	60	60	60	60
Net length	840	840	580	580	580
Net depth	240	240	260	260	260
Lot width	30	24	24	20	16
Lot depth	110	110	120	120	120
Lots per block	56	70	48	58	72
Parking – private	2	2	2	1	1
Parking – street	0.75	0.6	0.6	0.5	0.4
Driveway or alley	300	240	240	200	160
Pct. paved	8.33%	8.33%	7.69%	7.69%	7.69%
Back yard	960	768	1008	840	672
Pct. back yard	26.67%	26.67%	32.31%	32.31%	32.31%
ROW per unit	42.9	34.3	40.0	33.1	26.7
Density per block	23,129	28,911	26,136	31,581	29,403

Note: The density shown here is persons per square mile for the block itself.  Overall density as a whole would for a neighborhood of such blocks be much lower, since additional area might be devoted to such uses as larger roadways, parks, waterways, schools, libraries, places of worship, retail and service establishments, offices, hospitals, warehouses, and factories.

Some urban planners and urbanists criticize the long, narrow lots that are used in row house developments in older areas of American cities for being inefficient.  In a future post I will look at other high-density fee-simple lot configurations.

Planning: Parking and Garages

NB: I believe this topic has been discussed much better many times elsewhere on the net and in print.  I am writing simply to clarify my own thoughts.  I apologize for the primitive graphics.

Parking for personal vehicles is the bane of urban planners across the world.  The storage of these large, heavy items takes up a lot of space that can't be used for much else, even when the vehicles are elsewhere.  The surface of parking areas allows for little to no vegetation, depending on the type of material used.  Hard surfaces increases storm runoff, which slows the recharge of aquifers and raises the risk of flash flooding.  But people want cars for many reasons, and thus people want parking.

Personal vehicles are usually the first or second most expensive item a person or family purchases, depending on whether they are homeowners.  Given the amount of money at stake, the decision by many to store a vehicle under a roof is unsurprising.  Under protection, a vehicle will be dry most of the time, reducing corrosion.  A vehicle is also protected from weather damage due to storms, and from deterioration caused ultraviolet radiation.  Storing a vehicle inside also reduces the chance of it being stolen, vandalized, or damaged by a careless driver.  Trees cannot drop sap and birds cannot defecate on a vehicle under a roof.

Parking inside a structure makes getting in and out of the vehicle during inclement weather easier.  It also makes initial operation of the vehicle more pleasant, as vehicle will be cooler in sunny climates, and free of ice and snow in northern climates.  As more vehicles become electrified, having a convenient place to charge the vehicle without the risk of it being disconnected and the charging equipment stolen will be demanded by many people

Storing a vehicle inside is not without risk.  Vehicles can catch on fire, or leak unhealthy liquids.  And it can be expensive to create a separate structure or expand the main structure to accommodate vehicles.  A typical two-car garage is about 500 sq. ft., or from about 1/3 to 1/5 of the space in the living quarters in a typical single-family detached home.  In most cases, the amount of money saved by housing a vehicle in a structure will be less than the cost of the structure.  That is why a great many vehicles are parked outside on driveways.  Other vehicles spend most of their time in private parking lots, or parked on public rights-of-way.

Street parking is problematic for a number of reasons.  In many cases it is not priced directly, meaning there is no hourly, daily, monthly, or yearly charge paid by driver to park the vehicle which reflects the cost of the space.  Instead, the cost of building and maintaining the area on the street comes from taxes and fees.  Thus the incentive is for drivers to use street parking - especially in denser areas where most homes do not have provision for off-street parking - without any concern for how it affects the area as a whole.  Street parking also uses up large portions of the public right-of-way.  In historic areas, that can mean limiting sidewalk width to the bare minimum in order to accommodate empty, non-moving vehicles.  In newer areas, it means reducing the amount of taxable land and increasing the amount of hardscape.

The obvious response to those problems is to limit the demand, which is done in a number of ways.  Some jurisdictions require permits prior to a vehicle being street parked.  The permits usually come with a nominal fee, but the demand control is mostly accomplished by limiting their number.  Another way demand is limited is through metering.  Charging a sufficiently high price for a spot encourages turnover, which allows drivers to find spaces close to the location they wish to visit.  This is important for business establishments that depend on short (1/2 to 2 hour) in-person visits.  Often these metered spots revert to free parking overnight, and the area where they are install is always limited.  The technology used for metering has changed over the years, moving from mechanical coin-operated meters for individual spots to computerized pay-and-display machines for a large number of spots, but the purpose remains the same.

Another way jurisdictions limit demand for street parking is through mandating space for parking be set aside on private lots during building construction.  These parking minimums are controversial because they can greatly increase the price of a home and cause a spiral of reduced density and increased vehicle travel, resulting in the demand for more parking itself.  The minimums are often arbitrarily determined, forcing developers to overbuild parking in many circumstances.  Some jurisdictions have moved away from these standards, and have seen more housing built in downtown areas.

In stark contrast to American practice, Japan has implemented policies of neither requiring parking minimums nor providing for on-street parking in residential areas.  Any vehicle registered in Japan must have a documented place to park.  To catch vehicle owners that are dishonest about having a spot, many Japanese jurisdictions ban all on-street parking overnight, including in metered spots, though it isn't fully enforced until about 3AM.

While the goal of reducing sprawl is a worthwhile goal, parking still needs to be accommodated for the foreseeable future.  The best way forward is to decouple it from housing as much as possible in urban areas, reduce parking minimums wherever possible, and price public parking appropriately.

Planning: Block Sizes for Medium Lots with Alleys

NB: I believe this topic has been discussed much better many times elsewhere on the net and in print.  I am writing simply to clarify my own thoughts.  I apologize for the primitive graphics.

In a previous post I discussed the block size for lots that are of the scale found in older neighborhoods of detached houses.  An important conclusion is that by 40' wide, both front garages and curb cuts become a big issue.  One way to resolve the tension between the demand for private parking and curb cuts is to install an alley along the center of blocks.  (Alleys have a bad reputation as well as some technical issues that I will address in another post.)  Access to the rear of lots allows parking to occur there, and many homeowners choose to build garages on their property.

As you can see, the installation of an alley allows the entire front width of the house to be open, and eliminates the need for each lot to have a long, narrow driveway.  But it does increase the amount of hard area allocated to each lot over the short-driveway snout house described in the previous post.

	40' w/ short driveway	40' w/ rear garage	40' w/ alley	30' w/ alley	30' duplex w/ alley
Gross length	900	900	900	900	900
Gross depth	300	300	300	300	300
ROW width	60	60	60	60	60
Net length	840	840	840	840	840
Net depth	240	240	240	240	240
Lot width	40	40	40	40	40
Lots per block	42	42	42	56	56
Parking – private	2	7	2	2	2
Parking – street	0.5	0.5	1.0	0.75	0.75
Driveway or alley	297	1355	400	300	300
Pct. paved	6.19%	28.23%	8.33%	8.33%	8.33%
Back yard	2320	1380	1960	840	840
Pct. back yard	48.3%	28.75%	40.8%	23.3%	23.3%
Building separation	12	12	8	6	12

Note: all these houses have approximately the same floor area - not counting garage space, whether within the house or separate.  They are just shaped somewhat differently depending on the configuration.  Also, I have calculated the coverage percentages against a 120' depth even though the legal lot would be only 110' deep.

While adding an alley has clear advantages over front-driveway rear-garage homes, they actually somewhat less efficient in terms of hard area and providing for back yards than short-front-driveway front-garage homes.  Alleys increase the amount of on-street parking, and that parking comes in much larger chunks, meaning that longer vehicles such as moving trucks could be parked without blocking driveway access.  Alternatively, the on-street parking could be eliminated and the area used for bicycle lanes or wider curb lawns.

With a 40' lot width, in think the block length and depth remains workable at 900' by 300' when an alley is added.  The back yard is obviously smaller than with an 80' lot, but it is still a reasonable size even with parking taking a big bite out of it.  But at the next width down, the back yard shrinks substantially because the house becomes deeper (for the same area) and the space beside the garage basically disappears, being so small as to be not be useful for anything but a passageway.  A back yard of 28' by 30' would still be considered plenty in many areas of the world, but an argument could be made for increasing the block depth by 20' or 30', or 10' to 15' for each yard.

At a 30' lot width, another argument can be made that the fundamental form of the house should be changed as well.  So far I have assumed that the homes under discussion are single-family detached houses.  While that form dominates car-oriented suburbs - mostly due to legal reasons - in denser areas other forms start to appear.  These mixed neighborhoods were mostly built before comprehensive zoning became standard practice.  One form is the duplex (of the side-by-side type), or semi-detached house.  It is a fairly straightforward change from the detached house, not requiring a different form of ownership.  The legal structure supporting the party wall is well-established in most jurisdictions.  Sharing a single wall has the advantages of reducing the amount of energy needed to condition the home, and freeing up space for a larger side-yard, while still allowing light to enter on three sides of the building.

In a future post I will examine small lots, on which the rationale for connected forms of housing becomes overwhelming.