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.