The publication of a new research paper from the Joint Center for Housing Studies at Harvard University has provided policymakers with a useful nationwide assessment of different state-level policies regarding Accessory Dwelling Units (ADUs) in the United States.

The paper centers on two markets – New Hampshire and Portland, Oregon – as a means of comparing measured and more aggressive approaches (the latter having already been long-established as a national model). 

Researchers were seeking to combat the “self-reinforcing negative cycle” that is born out of patchwork regulations and has made market conditions even more challenging to their production. The results offer proof, according to the authors, that “robust changes can have dramatic impacts.”

Comparisons of the failures of different approaches to by-right approvals in creating new housing in the New England states is one key insight presented, along with three other key takeaways:

• 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.
• 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.
• 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.

The report will have bearings on the industry as it struggles to reconcile commercial demands with the needs of specific groups (seniors, low-income individuals, young people, etc.). ADUs are also seen as an important tool in the fight against housing segregation in states like Connecticut, where two-thirds of all 169 municipalities have decided to take opt-out provisions included in a progressive new law from 2021.

The report also assesses the boom of the ADU market in California, which went from seeing just 1,300 units permitted in 2016 to a total of over 25,000 in 2022 thanks to several proactive local and regional initiatives. Seattle is another growth new center with the pace of ADU construction recently outstripping that of single-family developments for the first time in history. 

Current Harvard GSD Master in Real Estate candidate Ellie Sheild and the Joint Center's Deputy Director David Luberoff are credited as co-authors. Their full, 52-page report ‘Accessory Dwelling Units Lessons for Massachusetts from Around the Country’ is accessible here.