Sunday, August 31, 2014

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. 


  1. This is a good discussion and the summary of Wegmann's thesis is useful. I'm wondering if the low rent obtained by Bridgeport's triple deckers is a result of the housing type, or the buildings' age and construction quality. Cheap wood-frame structures have poor acoustic insulation; you can hear your neighbors snoring at night, smell their cooking, and feel the building vibrate when they move. More solid and well-insulated multiplex houses do seem to enjoy stronger demand.

    Granny flats are never called duplexes because they are a different housing type. New urbanists have promoted and built small multifamily buildings and use the term "duplex" to mean two houses side by side sharing one wall. See:

    I hope in the future we'll see more small multifamily houses. Some places have even built new housing to resemble McMansions, for example:

    That particular housing type increases housing diversity in McMansion suburbs, but otherwise I see no advantages. The design is poor at all scales -- building, block, neighborhood. It has higher density without the urban design that makes density convenient and attractive. More urban expressions of the multiplex house are far more graceful and attractive additions to their neighborhoods, and (I would guess) maintain property value more consistently over the long term.

    1. Hi Laurence -- thanks for the comment. As for the reference to duplexes, what I was trying to convey is that the idea of granny flats, accessory units, etc. has been kept conceptually separate from the duplex form, although in both cases what is being described is essentially a second residential unit on the same property and often in the same building (I've seen apartments that are both vertically and horizontally separated, as well as in the classic "above the garage" configuration). In the CT towns I am familiar with, it is often possible to add an "accessory dwelling unit" to single-family houses, but the same is not possible in two-family zones. The only substantive difference between a duplex and a single-family house with apartment is a restriction on the size of the second unit (which in the example I am thinking of must be more than 450 sq. ft. but less than 40% of the size of the other unit, which often doesn't leave a very wide range), plus some aesthetic regulations so that it doesn't obviously look like a two-unit building. The property owner must also live in one of the units, but this I presume is difficult to enforce, particularly since there is no general requirement for SF owners to reside in the homes that they own.

      I guess my point here is that advocating for accessory/granny flats rather than for two-family zoning is really a political strategy to achieve what is a backdoor (literally, given some of the aesthetic regulations) upzoning of SF neighborhoods. The minor differences from two-family zones all appear to be means to address homeowner concerns. This is why I think NU's presentation of the concept, emphasizing either the subordination of the unit to the main house ("accessory dwelling") or the use of the unit by family, rather than strangers ("granny flat") is politically smart and has probably helped the concept to thrive.

      I agree about the importance of good urban design. A large part of the reason these 1970-1930 multifamily neighborhoods are so affordable, relatively speaking, is just that -- they have high densities without urban convenience. Just like the McMansion in the link, they are trying to conceal their higher densities rather than to work with and take advantage of them. The rowhouses of Brooklyn or Washington DC, often split into two (with garden apartment) or three units, don't suffer from this inferiority complex. Construction quality, especially for the wooden houses of New England and the upper Midwest, is definitely an issue too. The desire to built "flats" without good sound insulation results in substantial noise transfer (as I can personally attest).

    2. I agree. There is a physical difference, but in many ways, the political difference is more important. I live in Seattle, and unlike the East Coast, a lot of the multiplexes were built later (in the 1980s). There are a handful of older multiplexes (some very nice) but when most people think of multiplexes, they think of ugly buildings on ugly lots. Part of the problem is zoning, which required setbacks, parking and the like. All of that limited the ability of developers to make the nice looking buildings they made years ago.

      From a current political standpoint, Seattle is considering amending its accessory dwelling unit rules. Partly this is because they are more restrictive than similar cities (Portland OR, Vancouver BC) and partly because housing is in really high demand in this city. Despite that demand, though, no one (in power) has suggested amending the single family zones to allow for the conversion of houses to multiplexes, let alone the construction of new multiplexes. As you said, it is much easier to simply dangle that new carrot in front of the home owner (adding a new unit out back means more money for the property owner) rather than address the need for housing.

  2. The decline and then recovery of multifamily construction (>5 unit) a percentage of the overall market during the last decade exactly matches what happened during the 1991 recession. There is nothing new happening here.

    1. Well, yes and no I think? Even at the depths of the early 90s recession, the proportion of small multifamily (SM) declined only to 4.5% (in 1991), and actually rebounded slightly during the recovery (to 4.8% in 1995). In 2009, SM was at an all-time low of 3.5%, but did not recover and instead has continued falling (to 2.9% as of last year). As for 5+ multifamily starts overall, there overall pattern has been the same, but multifamily was at a higher baseline and has rebounded more sharply than it did in the early 90s (from 20%-34% from '09 to '13, as compared to 16%-20% from '91-'95).

  3. I don't have the historical correlation data to back this up, but it seems like one obvious new factor is condominium development. Three-deckers made a lot of sense to improve land use over single-family dwellings while still allowing a single owner-occupier to get a loan for the project. In the era of modern condo financing, what's the incentive for such a fixed division of owners and renters?

    1. That's a very good point -- although small multifamily construction almost disappeared during the 1970s and 1980s, that was the golden age for condo development. And of course condo units can be bought and held as investment properties as well, reducing the need for the rental unit to be on the same property or in the same structure as one's own home.

  4. Interesting how construction of small multi-family collapsed at the start of the Depression in Norwalk, because that's exactly when it took off in Cincinnati. There'd always been a handful of over-under 2-family dwellings, some even very cleverly disguised in imposing high-end houses in wealthy neighborhoods, but that changed when the Depression hit. We have a huge number of Art Deco 4-plex apartments, the majority of which were built in the 1930s and 1940s, with some going into the 1950s with a more discreet Colonial revival aesthetic. Some were built in new developments, others are mixed in single-family neighborhoods, and while they aren't entirely incognito they do fit in there quite well. They're all brick too, so they're relatively easy to fix up. There's no real history of 3-flats here that I know of though. The city seems to be trying to stamp the 4-plexes out of the zoning code, which is unfortunate, because they're perfect affordable housing that should be encouraged.

    1. Hi Jeffrey -- I performed the same exercise for the Cincinnati area (using a circular area drawn a bit outside I-275) and got the following figures for small multifamily buildings:

      1910-1929 -- 532
      1930-1949 -- 182
      1950-1969 -- 152
      1970-1989 -- 53

      So the overall pattern of decline is the same, but as you said there was still plenty of construction from the 1930s through to the 1950s, much more so than in Bridgeport/Norwalk at the same time. The peak decade is the 1920s in both cases, and the final drop-off occurs in the 1970s (and it looks like many of those from that era are actually incorrectly categorized by Zillow -- I did not go through them all individually).

      Interesting how the form of small multifamily buildings varies from place to place. There are relatively few four-plexes in the NE, although you do see them here and there. Way more often you'll see three-deckers joined together to create six-unit apartment buildings, like this:,-73.6349643,3a,75y,356.02h,89.58t/data=!3m4!1e1!3m2!1sX4ofqa4ZKK4CTPSGZsfM2A!2e0

      The three-deckers and four-plexes do fit into single-family neighborhoods quite seamlessly, and seem to work best in that context. Trying to build an urban neighborhood out of three-deckers, however, rarely produces a good result. It's not dense enough to support urban amenities, but due to the houses being packed very closely together, it feels much denser than it actually is.

      Bridgeport did get a few New York-style large brick apartment buildings before the city went into economic decline, but they are a tiny fraction of the building stock. They tend to be some of the best-kept housing stock in the city, though.

    2. I agree that whole blocks of these 4-plex buildings is a bit much. Design aesthetics aside, being all from the same era they deteriorate at the same rate, and as Jane Jacobs put it that's bad no matter the actual built form. It is rather unique though, which is interesting in and of itself.

      Looking back I see it as an early attempt to bring an affordable but inoffensive housing type out to the new leafy suburbs. Smash a few of these buildings together though and they don't look that different from some of the first public housing projects.

    3. Thanks for those links -- the built quality of these four-plexes is far superior to the vast majority of multifamily construction in Bridgeport. It's interesting, though, than in both cases they generally try to mimic the massing and siting of single-family homes, even where there weren't any single-family homes to blend in with.

      I'd agree with your first comment, and it's my impression that these structures have fared the best when mixed among housing of different types, with single-family, duplexes and triplexes all sharing the same street. Bridgeport was built up so fast, though, that no mixing occurred. Nonetheless, I think these neighborhoods are still more resilient than the single-family only neighborhoods from the 1940s and 1950s, which are particularly resistant to regeneration.

    4. "It's interesting, though, than in both cases they generally try to mimic the massing and siting of single-family homes, even where there weren't any single-family homes to blend in with."

      In the case of the 4-plexes, I'm sure it's because of the time in which they were built. By the 1920s there was a decidedly anti-urban propaganda machine in full force throughout the US. This also coincides with the anti-pedestrian, pro-automobile propaganda campaigns of AAA and others in the automobile industry. I bought a civic sciences book from 1923 at an antique store, and it completely eviscerates the city, even comparing the back sides of some rundown tenements with a brand new suburban (more like rural) ersatz farmhouse. "Which of these is a better home?"

      While the post-WWII era is often cited as the beginning of the suburbanization of America, it really got going in the 1920s. The Depression and WWII threw a monkey wrench into the works for sure, but the seeds of suburban housing development were planted and already sprouting before then. So it's no surprise to me that even multi-family developments at that time took on a decidedly suburban character. It's unfortunate because it manifests the worst of both worlds while achieving few if any of the benefits of either.

  5. It's interesting how the New England mill cities - Boston, Lowell, and Manchester, NH, for example - initially imported the British rowhouse, but then abandoned it and switched over to the triple-decker.

    Sometimes the massing and siting of triple-deckers can vary a bit from neighborhood to neighborhood. In some neighborhoods they're built very close together and right out to the sidewalk, so they feel more "urban," while in other cases - like the Bridgeport example above - they try to mimic suburban siting. Also keep in mind that many have been aesthetically "suburbanized" since the 1970s (stripped of ornament and slathered in vinyl siding) which makes them look even more like second-rate versions of SFDH these days.

    "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 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."

    While it's true that triple-decker neighborhoods seem to have gentrified slowly, they also seem to have experienced less "white flight" or ghettoization than urban neighborhoods in other formats (rowhouses or emphatically-urban apartment buildings). There are exceptions to both scenarios in Boston of course (where triple-deckers have, at different times, been trendified *and* slumlorded), but for the most part they haven't yet devolved into Wire-like slums.

    Still, I hesitate ascribing this stability to some magical property in the triple-decker typology itself, because I don't think that's the case. SFDH neighborhoods have turned into ghettos in the south and midwest, rowhouse neighborhood have turned into ghettos in the Mid-Atlantic, and apartment neighborhoods have turned into ghettos in NYC, so I don't think triple-deckers are immune.

    1. Hi Marc – sorry for the belated reply. I agree that there isn’t some magical quality inherent in this housing type. I would speculate that the small multifamily home is actually more resilient than the SF rowhouse or detached home in that it allows the owner (often the owner occupant) to sustain the cashflow necessary to keep the building maintained. The triple-decker neighborhoods of Bridgeport or Worcester often get very seedy and run-down, but rarely do you see mass abandonment on the scale of the rowhouse ‘hoods of Baltimore or the SFD areas of Detroit. Also, the presence of owner-occupants probably serves a stabilizing function within these neighborhoods.

      There may be a way to show that point mathematically, but I think it stands to reason that single-family dwellings by nature make poor rental properties, since the owner will almost always be an absentee landlord. Where a neighborhood “filters down” to a lower economic stratum, to a point where residents lack the ability to keep up a house of their own, single-family homes will gradually decay to the point of abandonment (and trying to rent out the house out to make ends meet would be like pulling yourself up by your own bootstraps, since you would simply be paying the same rent somewhere else). Not so for a three-decker – it absorbs falling rents by stripping ornamentation and substituting cheap materials, as you noted, but it remains structurally sound and inhabitable.

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