Through ReUrbanism, the Preservation Green Lab staff has focused its attention on the relationship between old buildings and the challenges and solutions for cities in the 21st century. Our team lives and works in cities—including Seattle; Denver; Washington, D.C.; and New York—and we’re tuned in to the conversations around livability, affordability, density, and displacement in our hometowns and across the country. One issue that seems to keep coming up and overshadowing other debates is density.

As Ed Glaeser argued in “The Triumph of the City,” cities with growing populations must provide adequate housing for incoming residents or they will face increasingly severe affordability challenges. Glaeser suggested that preservationists were partially to blame for the rising cost of housing, writing: “Construction restrictions tie cities to their past and limit the possibilities for their future.” But while preservation may restrict construction in some places, does it also protect and steward older places that provide dense, affordable space for homes, offices, restaurants, and retailers?

The Green Lab’s 2014 study, "Older, Smaller, Better," focused on the valuable contributions of old buildings in the cities of Seattle; San Francisco; and Washington, D.C.—not only those related to density and affordability but also those related to walkability, vitality, characteristics of jobs and businesses, and so on. In that study, we found a clear, statistically significant link between blocks of older, smaller, mixed-age buildings and heightened levels of population density. It turns out that older are remarkable in their ability to comfortably and inconspicuously fit incredible densities of residents, jobs, and businesses into relatively compact spaces. Perhaps the so-called “original design intelligence” of older buildings—a phrase coined by Cherilyn Widell but evidenced in writings by Stewart Brand, Carl Elefante, and others—can be applied to space-saving strategies for housing more people just as it can be applied to passive heating and cooling solutions that bolster energy efficiency.

The Green Lab’s newest product, the Atlas of ReUrbanism, allows us to delve into this subject further and explore a total of 50 American cities. To better understand the link between population density and the age of buildings, we recently analyzed the cities in the Atlas along these dimensions. To ensure a balanced comparison of old and new residential and mixed-use areas, we focused on 200-meter-by-200-meter sections of the city that contain at least 10 residents, excluding areas with large industrial and purely commercial uses. We queried the Atlas database for the average density of grid squares according to their median age of buildings, their majority prewar or postwar status, and the percentage of their stock that was built before 1920 and 1945.

The findings follow a clear pattern: areas with older buildings almost always have greater population density. In 43 of the 50 cities, the average population density in majority prewar areas was greater than in majority postwar areas. The other seven cities share a preponderance of postwar development and relatively scarce supply of majority prewar blocks, which makes the comparison a bit unbalanced to begin with. Meanwhile, in nine cities, the average density of prewar areas is at least 50 percent greater than the average density of postwar areas. These include some of the country’s largest cities (New York, Los Angeles, and Philadelphia), some of the country’s oldest cities (Boston; Providence; and Portland, Maine), and a set of cities with distinctive industrial legacies (Buffalo, Louisville, and Milwaukee).

An urban historian might astutely point out that postwar development took a more suburban pattern with culs de sac, detached garages, and large yards. Comparing prewar construction with this postwar suburban fabric could be construed as unfair. In many cases, however, it isn’t simply a matter of suburban single-family housing versus prewar apartment blocks. Building and zoning codes in many municipalities, for instance, have intentionally limited the density of new development, only allowing older buildings and their high densities to be grandfathered in. The New York Times recently ran a series of maps that showed the 40 percent of Manhattan’s buildings that would be illegal to build today, often due to their high density levels. To move past a simple prewar versus postwar dichotomy, we used the Atlas database to review density data according to median building age split into decades—comparing blocks with a median age in the 1920s against blocks with a median age in the 2000s, for instance). This revealed a similar trend. In Anchorage, Winston-Salem, and 37 other cities, the older blocks and neighborhoods have greater population density than areas dominated by new development.

To be clear: Areas with older buildings are not always denser, and these findings may not be so consistent in future decades. In my adopted hometown of Seattle, many areas with majority 21st-century development have greater population density than areas with majority early-20th-century development. I do not despair in this fact. (Though I will note that the spike of density in newly constructed areas may be due in part to four large, HOPE VI public housing developments that house lots of people in need.) In my view, Seattle is growing at such a rate that we surely do need dense new developments. Though older, smaller buildings play a critical role in supporting its local economy, walkable neighborhoods, and inclusive communities, Seattle has an opportunity to strengthen that existing fabric by building new buildings densely packed with people when the space is available. In a city with booming housing demand, building new structures on vacant lots and giving up some single-family housing for multifamily development makes good sense, and in fact, represents good preservation strategy. If we give a little, we may continue to keep a lot.

Mike Powe is the director of research at the Preservation Green Lab.