Missing middle is a term created by Opticos Design, a land use consultancy out of Berkeley, California. The term is used to refer to any low-rise construction denser than detached single-family housing:
The term refers to the fact that in many US cities, little of this housing is permitted. Instead, our cities jump straight from detached single-family houses to mid-rise apartments. The term is a bit ironic, as low-rise townhouses and low-rise apartments are one of the most common forms of housing worldwide and in pre-war US cities. They are only truly “missing” in newer US construction.
In places where land costs are high but single-family housing is required by zoning, allowing denser low-rise construction is a fantastic way of increasing affordability. Although construction costs can vary from type to type, missing middle housing tends to cost about the same to build per square foot as detached single-family housing, allowing land costs to be shared without adding additional construction costs. Indeed, low-rise housing can get dense enough that if enough of it was allowed, many cities could be built of nothing but low-rise housing without significant price pressures.
In some ways, it is only the fact that there is so little land devoted to apartments that makes mid-rise and high-rise construction so economically desirable. If dense low-rise apartments were allowed in the vast majority of land dedicated to single-family housing in growing US cities from Austin to Salt Lake City to Seattle, it may well be that prices would come down low enough that mid-rise housing would be much less economically viable. We seen this in our Restrictia model: when low-rise housing is restricted and mid-rise takes over, costs rise across the board to at minimum the cost of building a new mid-rise apartment.
This makes zoning that allows low-rise apartments in formerly single-family-only areas a fantastic first step. Rather than waiting for prices to rise high enough that mid-rise housing is economically viable, it addresses rising prices as soon as possible.
As wonderful as low-rise construction can be, it’s important to understand that there’s nothing magical about the form itself. At some land values and construction costs, it’s the most economically viable form of housing construction. Applied in these places, it allows more people to live in a place at lower prices. Applied elsewhere, it will have little effect. Already, in Austin, Texas, the focus on the gentle density design aesthetics of missing middle housing has allowed some anti-density advocates to suggest downzoning existing mid-rise areas to missing middle, rather than upzoning single-family areas to apartments. This would of course have negative, not positive, effects on affordability.