Several months ago, Charlie Gardner had an excellent, thought-provoking post asking why have American cities seen the demise of the duplex? In a time when growing cities are bursting at the seams and facing severe affordability challenges, an incremental kind of development might be welcome in many cities, offering new housing while allowing an evolutionary pace of change to a neighborhood’s physical fabric, instead of the abrupt transition of large-scale redevelopment. So why don’t we see more of it?

Consider international comparisons of small-scale incremental development: Charlie Gardner compares the built form on both sides of the US-Mexico border, noting how on the Mexican side houses grow incrementally over time, often adding new uses along the street. The net result is a slow transformation of the entire neighborhood, evolving towards denser development patterns. Gardner speculates on reasons for the difference with standard American development patterns (including finance and regulation), noting that the small-scale development open the door to homeownership at a much lower price threshold.

Conversely, there are examples of American neighborhoods adding units on a relatively small scale. Let’s Go LA has been tweeting highlights from Wallace Frances Smith’s “The Low-Rise Speculative Apartment,” published in 1964. The book documents the replacement of single-family homes with low-rise speculative apartments (often in the form of dingbats), concluding that this small-scale, relatively low-cost form of construction plays an important role in adding housing supply to the market. Without requiring challenging lot consolidation or more-expensive construction methods, this kind of incremental, small-scale development allowed neighborhoods of single-family homes to evolve into denser places – even without large incomes in the neighborhoods to afford expensive new construction.

Despite the small scale of each individual building, the net result was a substantial increase in housing production overall.

So, why don’t we see more of this today? While various New Urbanists might not like the specific dingbat product, the idea of small-scale urban density is still appealing. The so-called ‘missing middle’ forms, such as townhouses, flats, and small apartment buildings are all lauded as contextually-friendly ways to add housing and increase density in already developed areas. So, why are these housing types missing?

As Let’s Go LA points out, much of this kind of development has been regulated out of existence. In LA, large portions of the city have been downzoned; the newer zoning no longer allows for by-right development of dingbats and other small-scale apartment buildings. In aggregate, the result is a huge decrease in the potential development allowed in LA.

Much of that LA zoning potential would’ve been in the hands of small-scale landowners rather than large real estate development firms. One consequence of removing that development potential is to erode the ‘franchise’ for incremental development. Let’s Go LA notes that “by zoning small developments out of existence, we’ve made land development a much less democratic process, in the sense that far fewer individuals in the community are able to participate economically.” Instead, 20% of LA’s recent growth has been absorbed in the relatively small confines of downtown. While this is good for downtown (thanks to regulatory changes such as LA’s adaptive re-use ordinance and relaxation of off-street parking requirements – discussed previously here), limiting growth to such a small area of the city has consequences: “when growth is restricted across so much of the rest of the city, there will still be pressure on regional housing prices, and gentrification will continue.”

The phenomenon isn’t limited to LA or to dingbats. Stephen Smith, writing at New York YIMBY, looks at the demise of small-scale development (buildings smaller than five units) in New York: “Put simply: New York City’s small builders have been nearly eradicated. The segment of the market that normally produces about half the city’s new building stock has all but vanished.”

Smith considers several hypotheses for this decline in small-scale development, including the end of some tax abatement programs and weak markets in some parts of the city. Smith also hypothesizes that New York’s recent ‘contextual rezonings’ removed development potential from areas ripe for small-scale development:

The result is that many neighborhoods that were once full of redevelopment opportunities are now closed off to anything but the smallest of one- or two-family projects on vacant lots. This sort of redevelopment was largely banned after the implementation of the 1961 zoning code, but throughout her tenure Amanda Burden closed off the last few areas where it was still allowed.

DC is seeing similar conversations. Demand for additional housing often leads to ‘pop-up’ development, often in the form of vertical additions to existing rowhouses. The term even gets used as a catch-all for any kind of smaller scale infill development. Many existing residents are concerned about the changes (though others are supportive).

Responding to political pressure and resident requests, the Office of Planning proposed their own version of a contextual rezoning.However, during a hearing on the measure, one of the zoning commissioners expressed deep concern about the overall impact of reducing this development potential in a city with a growing population and decreasing housing affordability. Greater Greater Washington’s summary of the exchange captures the concern: “I just don’t think we have a comprehensive housing policy in this city and I’m worried about all the unintended consequences of [this proposal].”

While Charlie Gardner contrasted American urbanism to Mexico, there are other options as well. This paper from Sonia Hirt looks at German land use regulations. German zoning is guided by federal standards, localities have some flexibility within those standards but cannot add restrictions to the basic zoning classifications. One end result is that there is no such thing as a residential zone devoted solely to single-family homes. Likewise, even residential zones must accommodate commerce to meet the “daily needs” of the neighborhood.

In outlining potential routes for zoning reform in the United States building off of lessons learned from Germany, Hirt suggests that instead of relatively small areas of mixed-use zoning, planners could focus on a wider area of limited flexibility for residential development – something that might not look that different from the small, speculative apartment developments of the 50s and 60s; or of duplex development.