Research Shows Divergent ADU Policies Across the Country

“ADUs alone cannot fully address a host of housing-related challenges,” wrote a researcher from the Harvard Joint Center for Housing Studies, but she found they can be “an important part of meaningful comprehensive efforts to address them.”

In a report published late last year, Ellie Sheild, a graduate student researcher, compared several jurisdictions from around the country and their policies toward accessory dwelling units (ADUs). She identified three insights that could help inform policymakers as they consider regulating ADUs.

Described as small, independent living units that are inside, attached to, or detached but on the same parcel as a primary residence, ADUs can help address affordability, equity, sustainability, and other housing-related challenges.

Sheild found while some locales, such as Portland, Oregon, have embraced ADUs, other areas have “taken a more measured approach.”

Resistance to ADUs comes from homeowners who in turn pressure local jurisdictions to impose onerous restrictions, which JCHS researchers believe “can create self-reinforcing negative cycles in which few homeowners know about ADUs, few lenders offer appealing products to finance them, and few builders specialize in constructing them.”

In a blog post about states’ efforts to encourage ADUs, Sheild created a matrix comparing 11 states and which ones require by-right permitting, have a ban on owner-occupancy requirements, and have limits on parking requirements.

Sheild found several Western states, notably Oregon, Washington, and California, have passed laws that require by-right local approvals of ADUs and limit localities’ ability to impose arduous restrictions. Such laws can have significant impacts.

Portland’s annual permitting of ADUs grew from about 25 approvals in 2009 to around 300 in the past few years. In California, ADU permitting ballooned from around 1,300 units in 2016 to approximately 25,000 units in 2022, accounting for 18% of all new housing units.

By contrast, several New England states have passed ADU-related laws, but with more limitations. In Connecticut, for example, that state mandates by-right approvals but allows cities and towns to opt out of such requirements. Researchers found that two-thirds of Connecticut’s localities have taken advantage of the opt-out provisions of the state law. Vermont allows communities to impose occupancy restrictions and parking requirements.

Among insights Sheild listed that could be useful to policymakers when considering regulations pertaining to ADUs are:

1. Significantly increasing the production of ADUs requires the passage of statewide legislation mandating by-right, local approval of ADUs that meet meaningful state standards related to their size and design.
2. Policymakers must choose between a comprehensive approach that may limit communities’ ability to impose occupancy restrictions and parking requirements, or an incremental approach that does not include those limits. The former approach is likely to produce more units and more controversy; the latter will produce fewer units but might be more politically acceptable.
3. Meaningful by-right permitting processes will catalyze efforts such as homeowner education programs and new loan products that will help overcome other barriers to ADUs.

Sheild co-authored a 52-page report with David Luberoff titled Accessory Dwelling Units: Lessons for Massachustts from Around the Country.” It discusses potential benefits, such as economic, environmental, social, and health, for occupants, homeowners and cities, as well as major obstacles to the development of ADUs.

Sheild is a Master in Real Estate degree candidate at Harvard University’s Graduate School of Design, where she also holds a master’s degree in urban planning. Luberoff is deputy director at JCHS and author of numerous articles and case studies on the politics of infrastructure and land-use policies.

Want to learn more about ADUs in King County? Check the Education Calendar for one of our regular classes on the topic.