These two things are not the same. Left: a farm in Montgomery County by thisisbossi. Right: a house in Laurel by Eli Pousson. Image by the author.

Yesterday I wrote a post about single-family zoning in the Washington region. I got a lot of constructive feedback on the post, some of which was incorporated into revisions. Readers also wanted to know why I hadn't disaggregated undevelopable land, such as agricultural reserve, preserved open space, and federal lands from single-family zoning. Well…


Here's a second look at zoning in the Washington region, with particular thanks to contributors Chris Slatt and Payton Chung for helping me sort a few things out. I welcome additional tips from helpful readers, as there are still a few things that might not be right with this map.

For example, Fairfax County's zoning ordinance appears to not contain a zone for open space or agricultural reserve (though perhaps I am misinterpreting the R-A zone, which I did not include here as it allows single-family homes). See anything else missing?

Image by the author.

And here's the breakdown by the numbers:

Jurisdiction % Ag/Open Space % Single Family Total
DC 19% 23% 42%
Montgomery County 35% 48% 82%
Gaithersburg 0% 23% 23%
Rockville 10% 42% 52%
Prince George's County 35% 37% 71%
Laurel 0% 21% 21%
Arlington County 30% 42% 72%
Fairfax County 0% 77% 77%
Alexandria 6% 29% 35%
Falls Church 0% 40% 40%
Fairfax City 0% 54% 54%
Region 23% 51% 74%

You may notice that the Montgomery County total percentage changed from yesterday. I made an error in the denominator of the original calculation, using the area of the county *including* Rockville and Gaithersburg, even though the caption on the table said otherwise. These calculations have been double-checked and corrected.

And let's zoom back in on the urban core:

Image by the author.

And break that down:

Urban Core % Ag/Open Space % Single Family Total
Arlington 30% 42% 72%
Northwest 16% 36% 53%
Northeast 12% 15% 27%
Southeast 13% 9% 22%
Southwest 33% 0% 33%
All 21% 26% 47%

Recalling the original figure from Minneapolis, that 75% of the city was specifically zoned for detached single-family homes, we can now more accurately see that our urban core is in a similar situation, but for a different reason.

The large percentage of undevelopable land in the urban core and surrounding counties puts additional development pressure on the land that remains, and on jurisdictions that lie even further out, like fast-growing Prince William, Loudoun, and Howard Counties. In order to prevent that sprawl, we need a really ambitious vision for DC and the surrounding jurisdictions. To that end, I included the Metrorail network as an overlay on these maps, as requested by some readers.

I also added the Silver Line extension and the future Purple Line (though that will be operated by the Maryland Transit Administration and not by WMATA).