37. Do ADUs and zoning reform actually add housing?

Short answer: Yes, but how much depends on what the reform makes financially feasible to build, not on how many units it allows on paper. Results so far range from modest to substantial.

• Minneapolis became the first large U.S. city to end single-family-only zoning in 2020. Duplex and triplex construction stayed modest, averaging 57 permits a year for 2–4 unit buildings from 2020 to 2022 (Bipartisan Policy Center, citing Minneapolis Fed data). Most new housing came from apartment buildings along transit corridors. Rents rose about $150 less than in comparable cities, but the Minneapolis Fed attributes much of that to weaker demand after 2020. The advantage is now narrowing, with multifamily permits down more than 90% since 2022 as project economics deteriorated.

• Oregon (HB 2001, 2019): Across 45 affected cities, middle housing rose from about 8% to 11% of permits. Portland, which went further than the state law requires, now produces roughly 400 middle-housing homes a year, about a quarter of its permits.

• California ADUs are the clearest success. After state laws cut fees, parking requirements and approval times, ADU permits rose from fewer than 5,000 in 2017 to more than 23,000 in 2022.

  
  

Washington’s version. Washington passed its own versions in 2023: HB 1110 for middle housing and HB 1337 for ADUs. In King and Snohomish counties, three distinct policies often get lumped together as “zoning reform,” and they differ a lot in scale:

Capacity is not construction. Allowing more density does not guarantee more building. A 2026 study of Seattle’s Mandatory Housing Affordability (MHA) program found that permitting fell in modestly upzoned areas carrying affordability requirements, relative to nearby areas without them. The rare larger upzones, such as single-family to low-rise multifamily, increased permitting. The lesson is that new capacity has to be worth more than the costs a policy attaches to it.

What it means: Zoning reform is necessary but not sufficient; costs, fees and financing decide what gets built. ADUs are a useful incremental tool, while broad, economically feasible upzoning can produce much larger gains. Neither should be judged by units allowed. The real test is whether a policy makes the housing stock grow faster than it otherwise would have, and whether that added supply measurably improves affordability.