Everything homeowners need to know about building an accessory dwelling unit in Boston - size limits, owner-occupancy rules, permits, and why Boston plays by a different rulebook than the rest of Massachusetts.
Last verified: August 2026
6/10 · Moderately ADU-friendly
900 sq ft*
Max ADU size
Varies by zone*
Max height
Required
Owner-occupancy
By-right (internal)*
Permit type
This page summarizes Boston's own Accessory Dwelling Unit program, administered by the Mayor's Office of Housing and Boston's Inspectional Services Department, for general reference - it is not legal advice. Boston is not covered by the 2024 Massachusetts Affordable Homes Act's statewide ADU rule (see below for why). Regulations change; always confirm current requirements with the city before designing or applying for permits. See sources below.
Zoning & size requirements
What you're allowed to build
ADU type
Max size
Where it's allowed
Notes
Internal (basement, attic, in-unit conversion)
Up to 1/2 the primary dwelling's gross floor area or 900 sq ft, whichever is smaller
Citywide, on owner-occupied 1-, 2-, and 3-family lots
As-of-right: no Zoning Board of Appeal hearing required
Attached or detached (addition or backyard structure)
Same size formula as internal ADUs
As-of-right only in Mattapan; elsewhere generally requires Zoning Board of Appeal review
The city has signaled plans to expand as-of-right treatment to more neighborhoods over time
Why Boston is different from the rest of Massachusetts
Every Massachusetts city and town except Boston derives its zoning power from state law, MGL Chapter 40A - which is exactly the chapter the 2024 Affordable Homes Act amended to force by-right ADUs statewide. Boston instead operates under its own separate zoning enabling act dating to 1956, so that state ADU mandate doesn't reach it. Boston runs its own ADU program instead, built up through a neighborhood pilot and a series of zoning text amendments. Confirm your parcel's zoning subdistrict with the Boston Planning Department or Inspectional Services Department (ISD).
Additional standards
Setbacks, parking & other rules
Setbacks
Follow the dimensional standards of your specific zoning subdistrict, which vary significantly across Boston's neighborhoods - confirm with ISD for your address
Parking
Boston's own ADU rules don't add a parking requirement beyond what your zoning subdistrict already requires for the lot
Units per lot
1 ADU permitted in addition to the primary dwelling, on 1-, 2-, and 3-family lots
Owner-occupancy
Required - the property owner must live on the same parcel as the ADU. This is a real difference from most of the rest of Massachusetts, where the 2024 state law bars municipalities from requiring owner-occupancy
Building code
Must meet the Massachusetts State Building Code plus Boston's ADU-specific egress and safety standards
Historic districts
Boston's 9 local historic districts may require separate design review from the applicable historic district commission before ISD will issue a building permit
Ordinance history
How Boston's ADU rules got here
2019Original ADU pilot launches. Boston's first "Additional Dwelling Unit" pilot let owner-occupants of 1-, 2-, and 3-family homes in Jamaica Plain, Mattapan, and East Boston carve out internal units.
2023Mattapan goes fully as-of-right. The BPDA Board approved zoning changes in December 2023 allowing as-of-right ADU construction, including detached units, on all lots in Mattapan.
2024Zoning Commission adopts the Mattapan amendment; citywide expansion announced. The Boston Zoning Commission formally adopted the Mattapan as-of-right rules in January 2024, and Mayor Michelle Wu's State of the City address that year announced extending as-of-right internal ADUs to owner-occupants citywide.
2025Statewide ADU law takes effect - without Boston. Massachusetts' Affordable Homes Act ADU provision took effect February 2, 2025, but because Boston's zoning authority comes from a separate 1956 enabling act rather than MGL Chapter 40A, the city continues to operate its own program instead.
Worth knowing: city reporting has shown ADU permitting climbing as the citywide internal-ADU policy has taken hold, though total production is still modest set against Boston's housing needs. Mayor Wu's office has framed the program as a direct response to the city's housing costs, which rank among the highest of any major U.S. metro.
Permit process
How Boston ADU permitting works
1
Confirm eligibility
Check that your 1-, 2-, or 3-family home qualifies and that you'll live on the property. Email ADU@boston.gov or call the Mayor's Office of Housing with questions.
2
Design & feasibility
Budget up to about 3 months for this phase. Eligible owner-occupants can apply for a Technical Assistance Grant of up to $7,500 combined toward design and permitting costs.
3
Submit your permit application
Internal ADUs on owner-occupied 1-3 family homes go through Inspectional Services Department as-of-right, without a Zoning Board of Appeal hearing. Detached or attached projects outside Mattapan generally need ZBA review first.
4
Plan review
Complete applications typically clear zoning and code review in about 5 weeks; the full permitting phase can run up to 3 months.
5
Historic district design review (if applicable)
Properties in one of Boston's 9 local historic districts need sign-off from the district commission before ISD issues a building permit.
6
Construction & inspections
Construction typically runs 6-12 months depending on ADU type and scope, followed by standard framing, plumbing, electrical, and final inspections.
Fees, grant availability, and review timelines change - confirm current figures with the Mayor's Office of Housing before you budget.
Budgeting
What it costs to build an ADU in Boston
Internal ADU
Roughly $75,000-$100,000, per city program benchmarks - generally the least expensive path since it reuses existing structure
Detached ADU
Roughly $250,000-$350,000, per city program benchmarks, reflecting Boston's high construction costs
Technical Assistance Grant
Up to $7,500 combined, available to eligible owner-occupants to help offset design and permitting costs
These figures come from the city's own ADU program materials, not individual contractor quotes - actual cost depends heavily on lot conditions, finishes, and scope. Boston is consistently ranked among the most expensive U.S. markets to build in, so get multiple local bids before finalizing a budget.
Before you apply
Tips for a smooth approval
Know whether you're in Mattapan. It's the only neighborhood where detached and attached ADUs are as-of-right today - everywhere else, plan for possible Zoning Board of Appeal review unless you're doing an internal conversion.
Apply for the Technical Assistance Grant early. Up to $7,500 combined can help offset design and permitting costs before you've committed to a contractor.
Plan to live on the property. Owner-occupancy is mandatory for Boston's ADU program, unlike most of the rest of Massachusetts under the new state law.
Budget extra time in a historic district. Design review adds a step most other Boston ADU projects skip - check whether your address falls inside one of the city's 9 local historic districts before you finalize plans.
Don't assume state-law protections apply here. Because Boston runs its own separate program, provisions from the statewide Affordable Homes Act - like its transit-area parking waiver - aren't guaranteed to carry over.
FAQ
Common questions
Do I need to live on the property to build an ADU in Boston?
Yes. Boston requires owner-occupancy for its ADU program - the property owner must live on the same parcel as the ADU. This is different from most of the rest of Massachusetts, where the 2024 state law bars cities from requiring owner-occupancy.
Does the Massachusetts statewide ADU law apply in Boston?
No. The Affordable Homes Act's by-right ADU rule amended MGL Chapter 40A, and Boston is the one Massachusetts city that doesn't derive its zoning power from Chapter 40A - it operates under its own separate 1956 zoning enabling act. Boston runs its own ADU program instead, with its own rules.
Can I build a detached backyard ADU in Boston?
It depends on your neighborhood. Detached and attached ADUs are as-of-right only in Mattapan today. Elsewhere in the city, they generally require Zoning Board of Appeal review. Internal ADUs (basement, attic, or in-unit conversions) are as-of-right citywide on owner-occupied 1-, 2-, and 3-family homes.
Do I need extra parking for an ADU?
Boston's ADU rules don't add a parking requirement on top of what your zoning subdistrict already requires for the property.
What's the maximum size for an ADU in Boston?
Up to half the primary dwelling's gross floor area or 900 sq ft, whichever is smaller.
How long does the permit process take?
Complete applications for as-of-right internal ADUs typically clear review in about 5 weeks, with the full permitting phase running up to about 3 months. Projects requiring Zoning Board of Appeal review or historic district sign-off generally take longer.
How much does an ADU cost to build in Boston?
Per the city's own program benchmarks, internal ADUs typically run $75,000-$100,000 and detached ADUs $250,000-$350,000, reflecting Boston's high construction costs. Eligible owner-occupants can apply for a Technical Assistance Grant of up to $7,500 combined toward design and permitting.
Is financial help available for ADU design and permitting?
Yes. The city's Technical Assistance Grant offers up to $7,500 combined to eligible owner-occupants to help offset design and permitting costs.
Do historic districts have special ADU rules in Boston?
Properties within one of Boston's 9 local historic districts typically need design review approval from the applicable historic district commission before the Inspectional Services Department will issue a building permit.
