The Evidence Is In: 'Missing Middle' Zoning Reforms Are Successfully Expanding Housing Supply

The American housing crisis has long felt like an intractable math problem: too many people chasing too few homes, resulting in skyrocketing rents and locked-out first-time buyers. But over the past five years, a quiet revolution has been unfolding in residential backyards and municipal zoning boards.[8]

The concept of "missing middle" housing—structures like duplexes, triplexes, and Accessory Dwelling Units (ADUs) that sit between single-family homes and large apartment complexes—has moved from urbanist theory to concrete policy.[8]

California and Minneapolis serve as the primary laboratories for this deregulation experiment. By stripping away exclusionary zoning laws and streamlining the permitting process, these regions aimed to incentivize small-scale, "gentle density" development.[8]

The evidence is now rolling in, and the data suggests that these reforms are working. In California, the ADU boom has matured into a sustained driver of new housing supply. According to 2025 data from construction analytics firm Shovels, California permitted 24,192 ADUs, representing a 54.3% increase over a five-year period.[1]

This is not a marginal trend. In Los Angeles County, ADUs accounted for a staggering 38% of all new Certificates of Occupancy between 2022 and 2024, effectively becoming the backbone of the region's new housing pipeline.[7]

Further south, the Center for Housing Policy and Design at UC San Diego reported that ADU completions in San Diego County increased by 480% between 2020 and 2024. This explosive growth was directly catalyzed by state laws that forced local governments to approve ADUs on a ministerial basis, bypassing lengthy discretionary reviews.[4]

While California focused heavily on backyard cottages, Minneapolis took a broader approach. In 2018, it became the first major U.S. city to eliminate single-family zoning entirely through its Minneapolis 2040 Plan, allowing duplexes and triplexes on lots previously reserved for one home.[2]

A 2025 study published by the Global Labor Organization evaluated the five-year impact of the Minneapolis reform using a synthetic control method. The researchers found that the policy significantly suppressed housing costs: home prices were 16% to 34% lower, and rents were 17.5% to 34% lower, compared to a mathematically modeled "counterfactual" Minneapolis.[2]

The Pew Charitable Trusts corroborated this trend, noting that between 2017 and 2022, Minneapolis increased its housing stock by 12% while rents grew by just 1%. In contrast, the rest of Minnesota saw a 14% rent increase over the same period despite adding only 4% to its housing stock.[3]

Interestingly, the economic researchers noted that the price reductions in Minneapolis were not solely the result of a massive construction boom. Instead, the legalization of denser housing altered market expectations, softening speculative demand for single-family homes because the land could now be utilized more efficiently.[2]

A central question for policymakers is whether this new "missing middle" supply actually serves low- and moderate-income residents. The evidence here is nuanced but generally positive. ADUs are frequently described as "naturally affordable" because their smaller square footage inherently limits their market price.

Research from the Terner Center for Housing Innovation found that 58% of ADU owners rented their units at below-market rates. Furthermore, nearly a third of ADU residents were family members or friends of the homeowner, highlighting the role of ADUs in facilitating multi-generational living and informal care networks.[5]

However, market-rate ADUs are not a panacea for deep poverty. The UC San Diego study found that 69% of newly permitted ADUs in the county were priced for "above-moderate" income earners. Yet, even these units relieve pressure on the broader housing market by absorbing demand that would otherwise drive up the cost of older, existing apartments.[4]

Beyond the permitted construction boom, housing analysts point to a massive shadow market. Freddie Mac estimates that there are 1.4 million ADUs nationwide, a significant portion of which were built informally before recent legalization efforts. Bringing these existing units into the formal market remains a critical challenge for ensuring safety and tenant protections.[6]

The success of these early adopters is prompting a wave of similar legislation across the country, from Oregon to Massachusetts. The emerging consensus among housing researchers is that while upzoning alone cannot solve the entirety of the affordability crisis, it is a necessary prerequisite.

By legalizing the missing middle, cities are proving that the housing shortage is not an unsolvable law of nature, but a regulatory choice. The data from the past five years demonstrates that when homeowners and small developers are legally permitted to build, they will deliver the supply the market desperately needs.[8]