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.