Matt Yglesias and Ryan Avent both make the excellent point that since the District is the greenest part of the metropolitan area, the greenest thing it can do is work to increase its share of metropolitan jobs and people. On that point they’re totally correct. The best way to be more green is to get more people living and working in dense, walkable, and mixed-use places. Both Yglesias and Avent get off track, however, when they go on to suggest that the best way to accomplish that goal would be to raise the District’s height limit downtown.

The issue is that “dense” and “tall” aren’t the synonyms that Yglesias and Avent think they are.

There are two big reasons why tall buildings don’t necessarily equate to dense buildings:

1. Tall buildings almost always end up wasting vast amounts of space to oversized plazas and setbacks. If one compares aerial images of say, Ballston and Dupont Circle, it is easy to see that even though Ballston is fully urban and “built out” in its center, there are massive gaps in the urban fabric as compared with Dupont. This effect doesn’t stop at plazas; taken to an unfortunately frequent extreme it often results in cities where land owners think nothing of achieving the same density by building twice as tall and leaving half their land for parking. When land isn’t at a premium, there’s no reason to conserve it.
2. Tall buildings are difficult to modify. Avent makes the excellent point that it’s counterproductive to prevent large rowhouses from being subdivided into smaller apartments because subdividing them increases the number of units and therefore increases density. He’s right, but that sort of thing is much more difficult and rare at the elevator building scale than at the rowhouse scale, which means that elevator buildings are more likely to keep their original number of units over time rather than increase them. Since large new buildings almost always have to be “luxurious” in order to justify their construction costs, they’re almost always built with a lower number of expansive interior units rather than a higher number of small units. This means that generally speaking, tall buildings have fewer units on a per-square-foot basis than short buildings.

Examples of the non-synonym-ness of dense and tall abound. Ballston versus Dupont is one, but the quintessential may be Paris and its 11th arrondissement neighborhood. The 11th is the densest neighborhood in Europe and is comparable to the densest neighborhoods in New York. The vast majority of its buildings are shorter than those in downtown Washington. Very few are as short as two or three stories, but virtually all fall squarely in the category of midrises rather than highrises. Paris as a whole, for the record, is the densest city in the world outside of Asia, and except for La Défense (which only has about 1/3 the office space of downtown Washington) is almost entirely a midrise city.

Theoretically the two big problems with tall buildings listed above might be solvable with good regulations, but why bother? When you can achieve densities of over 100,000 people per square mile (such as those in the 11th) without buildings over 10 stories, why go to the trouble of trying to reinvent modern architecture and zoning? It’s just not necessary.

So far I’ve been talking mostly about residential density as opposed to job density. It is definitely possible to get drastically higher residential densities without increasing the height limit, but what about job density? It depends on where we’re talking. To get higher job density downtown, then yeah, we’d probably need taller buildings, but is that really what we need? “Downtown” isn’t a synonym for “central city” any more than tall is a synonym for dense. In fact, we’ve got buckets of underused land all over the central city just waiting to be redeveloped. In addition to places currently planned for office infill like RFK, Navy Yard, NoMa and Poplar Point, we’ve got the likes of that useless golf course in East Potomac Park and the acres upon acres of parking lot surrounding the Pentagon. And then there’s National Airport and Bolling Air Force Base. Ho boy! Imagine if we redeveloped those chunks of mostly empty land. They could *each* accommodate as much office space as we’ve got downtown now, just with midrises. All together, without going more than about a two mile radius from the Washington Monument or raising the height limit one inch, there is capacity to probably quadruple the amount of office space currently in the District of Columbia. If we kept the height limit downtown but raised it in Anacostia and Arlington, then that potential would increase even more.

So if it’s possible to drastically expand the density of the inner city in either way, skyscrapers downtown or filled-in gaps, which would be better for the city? While the skyscrapers downtown option would create a central office center surrounded by bedroom communities, each needing its own set of services, the fill-in-the-gaps option would create a larger number of fully mixed-use neighborhoods in which office and residential uses share services more efficiently. Both options require massive new investments in transit infrastructure, but while the skyscraper option would tend to support a single-hub commuter-oriented system, the infill option would support a more robust multiple-destination network more conducive to also serving non-commuting trips. While the skyscraper option would require massive inefficiencies in the form of tear-downs of existing downtown buildings (tearing down a 10-story building in order to build a 40-story one effectively only nets you a new 30-story building), the infill option would displace much lower-efficiency uses. While the skyscraper option would be a political nightmare unlikely to be well supported, virtually everybody wants infill at RFK (though that East Potomac Park suggestion may rile up some opposition).

Long story short: If we don’t need skyscrapers to be much more dense (we don’t) and there are good reasons to spread office growth around the central part of the region (there are), then why raise the height limit? (We shouldn’t.)

PS: For the record, I’ve got absolutely nothing against tall buildings. For five years I thoroughly enjoyed living on one of the highest floors in one of Ballston’s tallest buildings, and as the screencap of BeyondDC circa 2001 posted yesterday illustrates, this very website was originally created to track skyscraper development. I’m a fan of the skyscraper, in fact. They have definite pros and are without a doubt part of what makes an interesting, vital city. I think we should have much taller buildings in places like Silver Spring and Arlington, and fully support any proposals to make that happen. But it is simply false that tall buildings are a requirement for density, and false that the District must remove its height limit in order to accommodate growth.