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.