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.