2025 Boston ADU Laws and Regulations
The city of Boston is considered its own zoning district. For this reason, the statewide ADU law enacted under the Affordable Homes Act does not apply in the city. However, Boston has enacted its own ADU guidelines, which are both more and less permissive than the state law. We outline these differences below as well as what we think are the future of ADUs in the city.
Are ADUs legal in Boston?
Yes, some ADU’s are legal in Boston the way they are statewide but there are some key differences. The main difference is that detached or attached ADUs are not technically considered ADUs in Boston, but simply another dwelling on the existing lot. These units are being encouraged by the city to increase housing supply but are not considered by-right and require a special permit from the Zoning Board of Approvals.
Only internal ADUs within the existing building envelope (basement, attic, or carved out of existing space) are considered ADUs in Boston. The city is also requiring that the property be owner occupied which is not allowed on the state level.
The exception is in the neighborhood of Mattapan where the city is running an ADU Pilot program which is even more lenient than the new state regulation. These new bylaws allow internal, attached and detached ADUs to be built by-right on all R1 and R2 zoned properties. However, owner occupancy is still required.
The first key difference in Boston is the allowance of ADUs in multifamily properties. While only internal ADUs are by-right and owner occupancy is still required, the city is allowing these ADUs to be built in 2 and 3 family properties. This allowance is more permissive than the state wide regulation.
The other key difference is that in Boston is that when an ADU is built, it is considered a separate legal unit. For example, if you own a 2-family building and build an ADU in the basement, your property will now be considered a 3-family. This means that if the homeowner wanted to sell the ADU as a condo in the future, they would be able to do so.
Is this confusing? Yes, it is. That’s why we have created a table to outline the differences between Boston, Mattapan and the state.
| Location | Internal | Attached | Detached (including garage conversion) |
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| State (only single family zones) |
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| Boston (single AND multifamily zones) |
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| Mattapan (single AND multifamily zones) |
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Mattapan Pilot Program
A test program has been rolled out in the neighborhood of Mattapan. It has been communicated that the city plans to monitor its adoption through this one neighborhood before potentially expanding it to other neighborhoods.
Other than continuing to require owner occupancy, these pilot bylaws are significantly more permissive than even the new statewide regulations. If the city does decide to roll out this program in other parts of the city, it would materially increase the number of ADUs built.
Here is a summary of the pilot program with a link to the full details:
Boston has updated Article 60 to modernize residential zoning, including new provisions for Accessory Dwelling Units. The revised zoning allows by-right construction of ADUs on all lots, both non-detached (attached additions) and detached units. You can now add one non-detached ADU and one detached ADU per lot. The non-detached ADU may add up to 75% of the home’s existing square footage or 1,250 sq ft, whichever is smaller, and cannot exceed the height of the primary structure. A detached ADU is limited to 900 sq ft or the same footprint as the main house, is capped at 1½ stories, and must be at least 5 feet from any other building.
The Future of ADUs in Boston
Although in some ways, Boston ADU regulations remain less permissive than the broader state, the city does look to be moving in a direction to ease these restraints.
Based on our experience, it seems the city has realized they have a housing shortage and see ADUs as a way to improve the situation. They have taken a number of positive steps to promote ADUs and make their construction more accessible to homeowners.
With these initiatives, it is our opinion that Boston will continue to develop their ADU program and over time the restrictions will become more permissive. It is very possible that restrictions will be loosened beyond what we have seen at the state level. This would be great for homeowners who want to build an ADU for their family or income potential and to help combat the housing shortage that is affecting so many in the city.
Current Boston ADU initiatives
- Team dedicated to ADU development
The city has established a team dedicated to assisting homeowners with their ADU projects. In our experience they are responsive and happy to answer any questions you may have. They can be reached at adu@boston.gov
The team also holds a monthly design workshop on the first Thursday of each month where homeowners can present their ADU plans, ask any questions they may have and get feedback. The panel includes members of the ADU team, the Inspectional Services team and Fire Department, giving the homeowner different viewpoints on what is needed to make the project successful. You can register here:
- Mattapan Pilot Program
Explained above, this program looks to be a test program before a potential wider city roll out. If the city follows through with expanding these regulations to other neighborhoods and eventually the entire city, Boston ADU bylaws will be significantly more permissive than at the state level.
- Financing Programs
Boston is offering a Grant of up to $7,500 and an interest free loan up to $50,000 for qualifying applicants. These programs are offered for all ADU types in the city (internal, attached and detached) and in all neighborhoods.
Here are links to the facts sheets:
You can also find more information on how to finance on ADU on our Financing webpage:
- Extensive Guidebook
The city has provided an extensive guidebook outlining how to plan, design and build your ADU. The guidebook is more comprehensive than any other town in the state and is a good resource for learning what is possible for your ADU project.
The guidebook can be found here:
Boston ADU Zoning and Permitting Guide
Homeowners who want to build an attached or detached “ADU” (outside of Mattapan) will still need to get approval by the Boston Zoning Board of Appeals. Through our experience, we have created a step by step outline of what is involved so homeowners know what to expect.
Find our guide here or reach out to us with any specific questions or for help navigating the process here.
All Boston ADU Resources
- Review the City of Boston’s ADU Guidebook: Available online or for borrowing at any Boston Public Library branch.
- Register for our next ADU Design Workshop via Zoom. These monthly workshops (held the first Thursday of every month) allow homeowners to submit plans in advance and receive real-time feedback from City staff before applying for a building permit with the Inspectional Services Department.
- View the recording of our recent online ADU Information Session: Includes the chat transcript and slides available in English, Spanish, Vietnamese, and Haitian Creole.
- Explore the ADU Directory: Curated by the Boston Society of Architects to connect architects and architecture firms with homeowners looking to design and build ADUs.
- Learn more about financing options through the Boston Home Center:
- ADU Grant Fact Sheet
- ADU Loan Fact Sheet