Saturday, July 30, 2016

Build This: Musings on Small-lot Subdivision Design

Plot plan of a small-lot house with conventional setbacks

Small-lot houses have a lot of advantages.  They tend to be more affordable because they sit on less land than a typical detached house in the United States.  Their higher density means it is more cost effective to provide them with infrastructure (including more convenient transit service) and there should be more opportunities to walk and shorter driving distances if neighborhoods are designed on this model and mix land uses.  The lighter land footprint of small-lot houses means less natural habitat must give way to the bulldozer compared to conventional suburban development patterns.

I'd like to highlight a few things on the plot plan I drew above.  First of all, the lot size is only 3,500 square feet (325 square meters), which is pretty small by the standards of American suburbs.  Typically you see lot sizes of at least 1/8 of an acre (5,445 square feet or 506 square meters).  Local subdivision regulations often make the lot size above illegal to create because it is "too small."  This is a strategy for regulating against higher densities (and people of modest means).

I intentionally used conventional suburban setbacks on the plan, just to make the point that these houses could be tailored to fit into a conventional context in terms of how far they sit from the property lines.

Next, this design provides a two-car detached garage in the back yard.  As a practical matter, a two-car garage is often required by the zoning code.  The plan above shows that it can be done on a small lot.  The fact that the garage is detached is important on a narrow lot like this.  If the garage were attached, its driveway apron would be about 18 feet (5.5 meters) wide, meaning the curb parking space out front would be reduced to only 12 feet (3.7 meters), assuming a five-foot side setback (35' lot width, minus 5' side setback, minus 18' driveway apron equals 12'), much less than a standard 20-foot-deep (6.1 meter) parking space.  In the example above the apron is not shown, but it is intended to be nine feet wide and would exist just beyond the front property line and the sidewalk in the public right of way.  This design preserves 26 feet (7.9 meters) at the curb for one standard on-street parking space, plus six feet to roll out trash and recycle bins.  Attached garages also have other disadvantages, from car-oriented aesthetics to indoor air quality issues (if you aren't careful to properly ventilate them).  One of my beefs with San Francisco is how many of its buildings have driveways spaced so close together that large swaths of street parking, and even opportunities for street trees, are wiped out.  This starts to happen once the lot width gets down below 29 feet (8.8 meters) with nine-foot one-car driveways at the front.  This problem can be avoided through rear alleys, common driveways serving multiple units or dispensing with off-street parking altogether.

The width of the house is 20 feet, about the same as a townhouse.  Much narrower and the interior design starts to get awkward.  It would probably be two-stories in practice, since the market demand is for large houses, larger even than the 1,320 square feet (123 square meters) in the example above.  On this plan there is room to add on (assuming the zoning were designed to allow that).

Doing developments like the one above requires regulations that allow for smaller lot sizes and which regulate the width and spacing of driveways to preserve at least 26 feet of space at the curb in front of each lot (to have room for parking and trash bins at the curb and a parkway tree).  We know how to do this and the market for more affordable detached homes is definitely there.  The only question is, will local governments stand in the way?

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