Demise of the Duplex

The New York YIMBY website has complied Census building permit data to reveal how construction of single-family and small multifamily dwellings in New York City's five boroughs has plummeted since reaching a peak in 2004.  Of the potential explanations advanced for this collapse, contextual downzoning appears to the most likely to me, as the decline began four years before the peak permit year of 2008.  In general, however, small multifamily buildings have widely fallen out of favor not only in New York but across the country over the past thirty years and more.

Using the same Census data, this time calculating for all 50 states and the District of Columbia, it can be seen that small multifamily dwellings (those with between two and four units) fell from providing around ten percent of all new residential units in the early 1980s to a low of just under three percent in 2013.  Even as multifamily construction has rebounded since 2009, increasing its share of all units from 21 to 34 percent from 2009 to 2013, these smaller multifamily units have actually continued to decline as a proportion of the total (the chart shows the number of units, not the number of structures):

Triple deckers in Bridgeport, CT.

The permit data only begin to capture what is, I suspect, a much longer-term decline in this housing typology. Anyone familiar even in passing with the larger cities of the Northeast will immediately recognize the heavy predominance of the type in their older neighborhoods, as represented by the wood-framed triple-decker, or three-decker, house.  Whether built up to a flat roof, as in the typical image of the Boston triple-decker, or with the third story sheltered under a pitched roof, these are large and bulky structures that generally provide three spacious units with windows on all sides.  Despite the popular narrative of urban-dwellers fleeing cramped apartments for more spacious suburban homes, these units rivaled or exceeded in size the modest Cape-style single-family homes built in the 1940s and 1950s, were conveniently set on a single floor like the ranch houses that later became popular, and offered gracious architectural features such as bay windows and front porches that were often lacking in the new single-family homes. It would not be until the 1960s that the average new single-family home would significantly exceed the size of the ordinary triple-decker apartment.

The vertical axis shows the number of  two  and three-family
structures sold over the past three years within the metro area.

Census data provides housing statistics both by year built and by housing type, but unfortunately does not combine these statistics, making it impossible to determine what proportion of the small multifamily stock was built at what time. A workaround can be had, however, by using year-built information from housing listed on the MLS, which should provide a neutral sampling of the overall housing stock. Performing this exercise for the Bridgeport-Stamford-Norwalk area, an area with housing from all eras of American history and continuing high demand, reveals an exponential increase in the type from the 1860s to the 1900s, then a slower increase to an all-time peak in the 1920s.  Small multifamily construction collapsed during the Great Depression, along with most other housing construction, but unlike single-family construction did not rebound, even though the Bridgeport area experienced a manufacturing boom in the 1940s and early 1950s that attracted many factory workers.  Instead, it continued a gradual descent into irrelevance by the 1980s and 1990s.

The appearance of these structures therefore coincided both with the industrialization of American cities and a wave of immigration from rural America and from foreign countries, with the particular architectural style of multifamily housing in New England perhaps influenced or inspired by the multiplexes common in French-Canadian towns and cities (a building type which Urban Kchoze's Simon Vallee has recently explored).  These buildings had particular appeal to new immigrants, who could with sufficient savings purchase such a building and rent the upper two floors out to other immigrant families (often, members of their own extended family) to defray the cost of housing or perhaps even earn some additional income.

Why these dwellings ceased being built after the 1920s, never to return in any great numbers, is a difficult question, but fortunately it is not one that I need to guess at.  MIT graduate student Jacob Wegmann authored a 115-page thesis entitled What Happened to the Three Decker, supervised by none other than Sam Bass Warner, which explores that very question and offers a series of possible explanations which are equally applicable to other forms of small multifamily structures:

• Above all, exclusionary zoning, particularly in the 1970s and later, that restricted small multifamily housing from being built in those places where it would have been most desirable.
• Concentration of the real estate industry in the mid-20th century, resulting in the production of large-scale tract subdivisions that were able to exclude multifamily housing altogether.
• Federal involvement in mortgage finance starting in the 1930s and an emphasis on homeownership paid for by way of extended mortgage terms rather than over a shorter period with the aid of rental payments from tenants.
• Other regulatory barriers, including parking requirements, disability mandates from the ADA and state laws and enhanced fire safety requirements that have increased the construction cost for small, non single-family structures.
• A negative image of small multifamily dwellings that had always simmered among the native middle-class and which intensified during the 1920s and later, and which contributed to attempts to exclude these dwellings from newer areas of cities.

The last point, although it may seem less important, does indicate to me a genuine underlying problem with the three decker or stacked duplex form.  As an article on Worcester's three-deckers puts it, "to look at a three-decker means ... appreciating the attempts of three-deckers to echo freestanding single-family dwellings even in the midst of an undeniably urban setting and the effort to create an illusion of space for residents."  

Bridgeport, CT rent map showing lowest rents in duplex/
triple decker belt between downtown and outlying
single-family neighborhoods.  From Trulia.

That is, the small multifamily house was apologetically urban, and offered as its apology an attempt to mimic the outward form of the cottage or farmhouse style of housing prevalent in New England prior to the 1870s.  This reticence to adopt an unambiguously urban form left such structures appearing to be second-best, a characteristic that was not shared by rowhouse neighborhoods or those composed of larger apartment buildings.  The attempt to leave small gaps between buildings, rather than using shared walls, only served to emphasize the scarcity of space and lack of light and air as compared to larger lot single-family homes.  The virtues of having natural light on all four sides of a dwelling, however, were very much real and have been appreciated for decades by the residents of these apartments.

The limited appeal of these structures has had the upside of keeping them as relatively affordable housing options down the present day even in otherwise expensive cities.  Neighborhoods composed of them are not immune to gentrification, but their form and physical location -- typically in the inner-ring areas once served by streetcar lines -- combines to keep their rents among the lowest in the metro area.  

Although the building type has not come back into vogue, the notion of using a second residential unit on one's own property to help with mortgage payments has, through the New Urbanist revival of the so-called "granny flat" (a concept which goes by countless names, but never "duplex").  The strategy of appealing to homeowners' financial interests rather than to the need for more low-cost rental apartments is politically astute and has probably helped the idea gain traction.  In the meantime, in areas where demand is high, many single-family houses continue to undergo illegal conversion to multifamily use, indicating how the America of 2014, by banning the construction of small multifamily buildings and the division of homes into multiple units, once a commonplace process, is in some areas and in some respects doing an inferior job of housing the poor and recent immigrants than was the country of a century earlier.