“Missing Middle Housing” refers to a broad spectrum of smaller-scale multifamily building types that occupy the middle ground between single-family detached and large apartment buildings. At its edges, the term can cover rowhouses and 5+ unit apartments, but in my mind, the core of the “missing middle” are “plexes” — two, three, or four flats on a single city lot. Small apartment buildings like these line the side streets of most of America’s walkable urban neighborhoods: two- and three-flats in Chicago, Polish flats in Milwaukee, three-deckers in Boston, or brownstones in New York.
Census data show that 2-4 unit buildings have almost evaporated over the past generation. As recently as the early 1970s, over 1/4 of multifamily units built were in small buildings. Now, it’s only about 4%.
Despite the steep mid-1970s recession, over 10X as many units were built in small multifamily buildings as today. In boom years, 14X as many were built. It was during the Reagan era that these buildings broadly fell out of favor.
2-4 unit buildings have been a relatively small share of all multifamily construction in the postwar era. Since the 2008 recession, they’ve practically disappeared — with only a few thousand units built nationwide every year:
Next up: Wait, you mean today’s apartment boom isn’t actually that big?
west north 
I wonder if the “missing middle housing” is reflective of reduced consumer demand for this housing/building type. 2-4 unit multi-family buildings aren’t large enough to support the amenities found in larger (i.e. very large) multi-family developments. They also aren’t single-family homes (detached or attached) with all the “trappings” of owning your own lot. Also, from an ownership prospective, this small-development scenario runs the risk of pitting 2-4 households against one another as they disagree over building upkeep. As was once told to me, a condo buyer should pursue a large-ish building for “sanity in numbers” to avoid the log-jam that might occur with only 2-4 owners in a building.
On the whole, great post…I’m simply surmising.
Small multi-family buildings would be a good way to gradually intensify the use of centrally located single family neighborhoods. But neighborhood associations in such areas typically demand and often get single family only zoning, or zoning that as a practical matter eliminates small apartment buildings. I feel pretty confident that if zoning allowed these buildings, architects, planners and even bankers could figure out how to bring them into being.
@ Chris M . I think it’s a good question that you’re asking. Based on my experience in Cambridge, where there is very high demand for small multifamily housing (as for housing in general), I’d hypothesize that the main thing driving the drop has been wealth inequality combined with regulatory burdens. Small projects have very high regulatory barriers to creation in most municipalities, relative to the potential gain, but if you’re a small developer you may only have the capital to engage in a smaller project. During the recession, and over previous decades, I’d guess the number (or potential) of small property owners/developers declined.