The Evidence Behind 'Missing Middle' Zoning Reforms: Do They Actually Lower Housing Costs?

The housing affordability crisis has forced cities to rethink the American dream of the detached single-family home. For decades, restrictive zoning laws effectively banned the construction of duplexes, triplexes, and small apartment buildings—often termed "missing middle" housing—across vast swaths of urban land.[6]

Proponents of zoning reform argue that re-legalizing these moderate-density structures is the most effective way to increase housing supply and stabilize rents without dramatically altering neighborhood character. Until recently, this was primarily a theoretical debate.[6]

Today, a growing body of empirical evidence from early-adopter cities provides a clearer picture of what actually happens when municipalities abolish single-family exclusive zoning. The data reveals that while upzoning can successfully tame rent growth, simply changing the law is not a magic wand.[6]

The strongest evidence for the supply-side argument comes from Auckland, New Zealand. In 2016, the city upzoned approximately three-quarters of its residential land to allow for more intensive housing.[2]

A rigorous quasi-experimental study published in the Journal of Urban Economics found that the policy triggered a massive construction boom. Researchers concluded that the upzoning resulted in over 10,000 additional dwellings being built over six years, primarily attached multi-family units.[2]

Similar supply responses are emerging in North America. In Portland, Oregon, the city implemented the Residential Infill Project in 2021, allowing up to four units on nearly all residential lots. According to researchers at the University of Toronto, missing middle housing jumped from 13.4% of total permits to 44.7% in the two years following the reform.[4]

The most closely watched American case study is Minneapolis, which in 2020 became the first major U.S. city to eliminate single-family zoning citywide through its 2040 Plan.[5]

An analysis by the Pew Charitable Trusts found that between 2017 and 2022, Minneapolis increased its housing stock by 12%, while the rest of Minnesota managed only a 4% increase. The influx of new units had a profound impact on affordability.[1]

During that five-year period, rents in Minneapolis grew by just 1%. In stark contrast, rents across the rest of Minnesota surged by 14%. Pew researchers estimate that this moderation saves the average Minneapolis renter roughly $1,700 per year compared to the statewide trend.[1]

The Auckland data corroborates this price-moderating effect. Economists found that six years post-reform, rents for three-bedroom family homes in Auckland were 22% to 35% lower than they would have been in a synthetic control scenario without the upzoning.[2]

Furthermore, the increase in housing supply correlates with broader social benefits. While homelessness rose by 14% across the rest of Minnesota from 2017 to 2022, it actually declined by 12% in Hennepin County, which includes Minneapolis.[5]

However, while Minneapolis and Auckland showcase the potential of zoning reform, California's experience highlights the friction of implementation. In 2022, California enacted Senate Bill 9 (SB 9), a landmark law allowing homeowners to split lots and build up to four units on single-family parcels statewide.[3]

Initial projections suggested SB 9 could make 700,000 new homes financially feasible. Yet, a review by the Terner Center for Housing Innovation at UC Berkeley found that actual uptake in the first year was minimal.[3]

Los Angeles, a city of nearly four million people, received just 211 applications for new units under SB 9 in 2022. San Diego reported a mere seven applications. The Terner Center identified multiple barriers preventing homeowners from utilizing the law.[3]

High construction costs, elevated interest rates, and a lack of development expertise among everyday homeowners severely limit the financial viability of lot splits. Furthermore, local municipalities often impose restrictive design rules, height limits, and maximum unit sizes that render SB 9 projects mathematically impossible to build.[3]

The evidence suggests that for missing middle housing to scale, zoning reform must be paired with streamlined permitting and a reduction in bureaucratic friction. When municipalities actively support the transition—as seen in Minneapolis's elimination of parking minimums—the market responds.[1][5]

Ultimately, the data indicates that ending single-family zoning is a necessary but insufficient condition for solving the housing crisis. While it provides a proven mechanism to stabilize rents and increase density, realizing its full potential requires aligning financial incentives, building codes, and local political will.[6]