Yes, but mostly inside your house. As of October 2026, Boston is not covered by the state's by-right ADU law. Its own zoning lets owner-occupants create an ADU within existing space, such as a basement or attic, without exterior changes. Detached ADUs are allowed by right in Mattapan, and draft zoning would add them in Hyde Park, Roslindale and West Roxbury.
Why doesn't the state ADU law apply in Boston?
The Affordable Homes Act changed the state Zoning Act, Chapter 40A, and Boston's zoning runs under a separate enabling law. Banker & Tradesman called the result "a quirk in state zoning law," and the Pioneer Institute notes that Boston is not covered by the statute. Boston's rules therefore come from its own zoning code.
What can I build in Boston today?
- An interior ADU, citywide. Owner-occupants can create a unit within existing space, with no exterior alterations, according to the Boston Planning Department. Boston started with a pilot in 2017 for owner-occupants of one-, two- and three-family homes and later expanded it.
- A detached ADU in Mattapan. Mattapan's residential zoning was amended to allow detached ADUs by right on owner-occupied lots.
Anything else, such as an addition or a backyard cottage outside Mattapan, generally needs zoning relief from the Zoning Board of Appeal.
What is changing in 2026?
On July 17, 2026, Boston released draft "Neighborhood Housing" zoning for Hyde Park, Roslindale and West Roxbury. It would allow detached ADUs by right in three residential districts and as a conditional use in four others. The public comment period runs until October 31, 2026, and the Zoning Commission must then approve it. The Pioneer Institute reports that similar changes in other neighborhoods are unlikely to start before 2027.
How many ADUs has Boston permitted?
The Mayor's Office of Housing reported 212 ADUs permitted between 2019 and July 2025, as cited by the Pioneer Institute in August 2026.
Is there help for Boston homeowners?
The Boston Home Center and the Mayor's Office of Housing offer ADU financing support and technical assistance. The city has also published an ADU guidebook of sample designs; the Planning Department notes these are not pre-approved plans.
When does the Boston answer depend on your street?
It depends on your neighborhood, your zoning subdistrict and whether you live in the house. A Mattapan owner-occupant has a by-right detached option today. A Roslindale owner may have one soon if the draft is adopted. An investor-owned house elsewhere has fewer options under current rules. We check all three before you spend money on design.
Related
Sources, as of October 2026: Boston Planning Department, Citywide ADU Zoning; City of Boston, Additional Dwelling Units program history; Pioneer Institute, The House Call (August 2026); Banker & Tradesman (May 13, 2026).
Updated October 2026