Infill Housing Has Its Benefits but Won't Always Drive Down Costs

There's a school of architects and urban planners who support allowing more housing in existing neighborhoods, if new structures are similar in size and appearance to existing single-family houses. Call it the “compatibility school.” They support rule changes that generally allow for somewhere between two to six units to be built on sites with roughly the same footprint as single-family houses in the same neighborhoods.

This approach can deliver beautiful new homes in highly desirable locations. But it hasn’t proven to be a way to deliver a significant amount of new housing.

Charleston, S.C., provides perhaps the country’s best example of historic development patterns that are successfully accommodating compatible infill projects. In fact, prominent architects from the compatibility school were involved in shaping the land-use restrictions that are in place in Charleston today.

Charleston’s pattern of tall “single houses” on narrow lots provides a development pattern of around eight large houses per acre. Often these large houses are split up into smaller apartments, leading to higher unit density.

New construction blends seamlessly into Charleston’s historic patterns. Many of the buildings on the peninsula predate the car, offering a walkable development pattern. Flexible rules allow for new lots to be added to alleys on existing blocks, creating whole new streets and added interest for passersby who can view layers of houses and subtropical gardens while also increasing the number of households that can support neighborhood retail.
Even under Charleston’s relatively dense baseline zoning, some of its most celebrated new construction happens under planned unit developments, a planning tool that allows flexibility, but which often comes at the cost of extended delays and uncertainty. The city’s much-vaunted Catfiddle Alley is one such example. It includes relatively small, mostly three-story structures that all look like houses, but many of them include more than one unit. Its density is an impressive 40 units per acre.

While Charleston is gorgeous, it’s no model of affordability. This is due to many factors, including a spike in demand for both primary residences and vacation homes. But its regulations don’t help either. Projects on the peninsula must go through a long and uncertain review by the city’s Board of Architectural Review. This may be one source of the high quality of architecture that makes the city a darling for urban planners, residents and tourists. But this review, combined with a shortage of space where large multifamily projects can be built, contributes to serious housing supply and affordability challenges.

The Charleston model is not delivering a lot of housing, but it does show that new construction can contribute to beloved historic neighborhoods that already offer the benefits of urban, walkable living. The recent trend of moving away from single-family zoning applies an approach akin to Charleston’s — allowing new construction of a similar scale to existing buildings — in places that lack its pattern of walkable urbanism. In general, these reforms are not facilitating construction proportionate to the level of public debate and controversy they have caused.

Minneapolis and Arlington County, Va., have both recently adopted “missing middle” zoning reforms. This policy, shaped by the compatibility school, allows multiple units within house-scale structures in neighborhoods previously zoned for exclusively single-family zoning. Relative to Charleston, single-family houses in Arlington or Minneapolis sit on large yards. They have deep front and side lawns separating them from their streets and from their neighbors. This space contributes to quiet and privacy, but it also reduces the walkability of these neighborhoods by creating longer distances and dead space for someone on foot to traverse between destinations. It also prevents much neighborhood retail due to low population density.

Allowing a few more units to be built in this type of house-scale structure can offer cost savings to potential residents. But it also asks them to trade off space and privacy without getting the benefits of walkability that a place like Charleston offers. Allowing smaller units within the scale of a typical U.S. single-family neighborhood is never going to get close to the density of Catfiddle Alley.

Outside of their single-family zoning reforms, both Minneapolis and Arlington have been national standouts for permitting large numbers of big apartment buildings. In both places, rents are more affordable than in peer cities, in part due to this multifamily construction. One new apartment building in either locality may exceed its number of missing-middle units permitted in a year.

Debates over replacing single-family zoning with missing-middle zoning galvanize both supporters and opponents of new housing supply. It’s possible that on balance, raising the prominence of zoning in local policy debates will ultimately fuel awareness of the need to allow housing to be built, providing support for other reforms that result in serious levels of new construction. But there’s no way around it, reforms that lead to ample new construction will result in more built square footage in addition to more units.