American urbanists are starting to realize there’s a problem with single-family-exclusive zoning. But is it a problem in DC? For a thought experiment, let’s turn it around: what if all residential land in the District allowed nothing but single-family homes?

For this post, we’re using “single-family zoning” to refer to laws allowing nothing but single-family detached homes to be built in a certain place. Such laws mean that, in order to build literally anything other than a single-family house, you have to seek permission.

In DC, laws limit the number of multi-family homes that can be built. Making it illegal to build duplexes, rowhouses, and apartments in parts of the District means that we have fewer homes here than we might otherwise. This makes housing more expensive: There’s less of it when there clearly needs to be more—particularly studios and one-bedrooms—to meet the needs of both longtime residents and newcomers.

To understand the inefficiencies inherent in single-family zoning, we engaged in the following thought experiment: What if the entire city (or, at least, all of the residential areas) were zoned only for single-family detached homes?

Image by the author.

DC has two types of single-family detached house zones: R-1-A, for larger lots, and R-1-B, for smaller ones. Only 5% of residential land in the District is zoned R-1-A compared to 26% R-1-B. Plus, “detached houses on moderately sized lots” practically means separated single-family homes with little yards. That, we think, is a reasonable description of what occupies most people’s imaginations when they think of the sorts of neighborhoods that, some argue, need to be defended against encroaching density.

Besides R-1-A (detached houses on large lots) and R-1-B (detached houses on moderately sized lots), DC has two other single-family house zones: R-2 (semi-detached houses, where pairs of homes share one wall but aren’t in rows) or R-3 (attached rowhouses). Most row houses in DC aren’t R-3 but RF (“residential flats”), which allow two units per house, but a few areas including much of Georgetown is R-3. We chose R-1-B for this analysis because it’s a fair and interesting benchmark.

The minimum lot size for R-1-B is 5,000 square feet. In a very simplistic analysis (that in reality would involve re-platting all the residential land in the city and evicting institutional land uses like the Naval Observatory), DC contains just over 1 billion square feet of land zoned residential, so dividing that number by 5,000 square feet gives space for 211,323 R-1-B homes. The 2013–2018 American Community Survey five-year estimate for the number of housing units in the District is 319,579. So if we only housed people in R-1-B-type housing, we’d lose about a third of the homes here.

Roughly 31% of residential land in the city is designated R-1-A or -B, including (for example) most of the AU Park, Kalorama Heights, and Woodridge neighborhoods. Those neighborhoods, of course, have duplexes and apartment buildings in them. But should you wish to build a new one — it doesn’t matter if it’s a duplex or an eight-story condo building—you must ask for special permission from the Board of Zoning Adjustment or the Zoning Commission.

Banning structures other than single-family detached houses from 31% of the District’s residential land brings consequences. Multifamily housing allows more people to live on a given piece of land than a single-family house does. And the footprint of a single-family house is often considerably larger than the dwelling itself, because other regulations define the smallest size a lot can be and how close the structure can be to the road.

When we mandate low-density, single-family, detached homes in places where lots of people want to live — or where lots of people could live so that they could more easily access those jobs and amenities — it becomes more expensive to live here. It’s bad for the climate, too, and it’s at odds with the District’s and the region’s stated sustainability and housing goals.

Let’s look at this by ward

It’s also interesting to think about this on a ward-by-ward basis. All of the R-1-A land in the District is in Ward 3 and the westernmost edges of Ward 4, with the exception of a really tiny piece of Embassy Row in Ward 2. But 16% of the residential land in Ward 4 is R-1-A, and 10% in Ward 3, so our R-1-B scenario is actually an upzone of those wards.

For every other ward in DC, zoning all residential land R-1-B is a substantial downzone. If we only housed people on R-1-B, we’d lose the beloved rowhouses of neighborhoods like Capitol Hill and Petworth, the stately apartments and coops lining Connecticut Avenue in Northwest, the glass-and-steel high-rises of Southwest, and the garden apartments of Congress Heights.

Square feet Zoned R-1-A Zoned R-1-B Zoned residential How many

R-1-B houses Ward 1 70,696,445 54,113,416 10,823 Ward 2 242,109,498 646,953 5,170,052 51,386,721 10,277 Ward 3 304,841,116 22,104,591 107,718,792 231,611,736 46,322 Ward 4 250,958,728 28,760,468 72,527,989 176,884,799 35,377 Ward 5 289,663,864 53,497,648 177,668,864 35,534 Ward 6 172,920,633 82,785,071 16,557 Ward 7 245,611,890 36,747,883 160,889,215 32,178 Ward 8 333,312,329 121,273,590 24,255 DC 1,910,114,503 51,512,011 275,662,364 1,056,613,411 211,323

So what did we learn?

Suppressing multifamily housing doesn’t mean curbing renters in a neighborhood, or density for that matter. It simply means, in many cases, that families will compete for existing housing stock with unrelated individuals who have organized themselves into group quarters because of a lack of single-family-style alternatives. This only exacerbates housing affordability challenges for families.

This exercise illustrates the value of “gentle density”. We wrote recently, “Removing barriers to townhomes, two- to four-family homes, and small-scale multifamily buildings in every part of the city” would help considerably in meeting the District’s housing needs.

Much of the debate around development in DC is about upzoning to the highest-intensity, mixed-use zoning categories. What if we tried letting 1,000 duplexes, townhomes, and low-rise apartment buildings bloom?

Just as we imagined removing these types of homes and, in doing so, lost a third of our housing inventory, allowing these “missing middle” housing types on 31% of the residential land in the District could dramatically boost the number of places for people to live. Density is critical to that equation; single-family zoning, like R-1-B, makes it impossible to build more, smaller homes at a lower cost.