In Search of the Missing Middle: Historical Trends in and Contemporary Correlates of Permitting of 2–4 Unit Structures

Missing middle housing is an important although often overlooked housing form in America’s built environment. Although still a large component of the US housing stock, production of new small missing middle (SMM) housing—which we define as multifamily structures with two to four units—has steadily declined over the last several decades. In the early 1980s, units in SMM structures comprised ∼9% of residential building permits nationally. Today, less than 3% of new permits are for two- to four-unit structures. We document these trends and explore reasons for the current variation in new SMM production. We build a national, place-level data set combining data on building permits, current and historical census data, land-use regulations, and housing supply characteristics. We then examine the association between SMM production and three sets of community characteristics: supply, regulatory restrictiveness, and demand. Our analysis suggests that correlates of SMM permitting are similar to those for larger multifamily structures. This, we argue, helps explain the decline in SMM, because these developments are competing with and losing to larger multifamily development. We end by considering how these findings can help cities that have recently passed or are considering zoning reforms that broadly legalize missing middle housing forms.