Can I Build an ADU in Massachusetts?
Yes. Since 2 February 2025, nearly every Massachusetts homeowner can build one accessory dwelling unit by right in a single-family zoning district, with no special permit and no public hearing. The unit may be up to 900 square feet, or half the gross floor area of your house, whichever is smaller. The change came from the Affordable Homes Act, Chapter 150 of the Acts of 2024, implemented through 760 CMR 71.00.
That is the short answer. The rest of this explains the size math, which is where most projects get redrawn, and the handful of things your town can still require.
The 900 square foot rule, and the word most summaries get wrong
The cap is 900 square feet or half the gross floor area of the principal dwelling, whichever is smaller. A number of published guides say "whichever is greater." That is wrong, and it is an expensive error to carry into a design meeting.
| Your house (gross floor area) | Half of it | Your actual ADU cap |
|---|---|---|
| 1,200 sq ft | 600 sq ft | 600 sq ft |
| 1,400 sq ft | 700 sq ft | 700 sq ft |
| 1,800 sq ft | 900 sq ft | 900 sq ft |
| 2,600 sq ft | 1,300 sq ft | 900 sq ft (capped) |
| 4,000 sq ft | 2,000 sq ft | 900 sq ft (capped) |
The practical threshold is 1,800 square feet. Below it, your house size sets the cap. Above it, the 900 square foot ceiling does.
How gross floor area is measured
This decides the number above, so it is worth getting right before anyone sketches anything. Measurement is taken from wall centrelines, not interior faces.
Counts: basements, lofts, and intermediate floored tiers.
Does not count: crawl spaces, garage parking areas, attics, and enclosed porches.
If your lot has more than one principal dwelling, the largest one sets the limit.
Below 1,800 square feet, your house sets the cap. Above it, the law does.
What your town can no longer require
For a first protected-use ADU, a municipality may not:
- Require that either the ADU or the main house be owner-occupied
- Require a special permit or public hearing
- Impose a minimum lot size for the ADU
- Cap or quota how many ADUs exist in the town or district
- Dictate whether the unit is attached or detached
- Require more than one additional parking space, or any at all when part of the lot is within half a mile of a transit station
- Apply dimensional standards stricter than those applied to the principal dwelling
- Apply subjective design review
Two narrow exceptions survive: a town may still require a special permit for an additional ADU beyond the first on the same lot, and within floodplain or aquifer protection overlay districts.
Where the law does not reach
Boston is not covered. The city zones under its own enabling act rather than M.G.L. c. 40A, and runs a separate programme.
More importantly, the ADU regulation does not override the state building code, the fire code, wetlands protection, or Title 5 septic rules. That is where nearly every real-world constraint actually lives.
The thing that stops more projects than zoning
Wastewater. On an unsewered lot, Title 5 allocates 110 gallons per day per bedroom. A three-bedroom house with a system designed for 330 gallons per day has no headroom. Adding a one-bedroom ADU means 440 gallons per day, which means a system upgrade, which on a difficult lot can cost more than the ADU.
Zoning is now usually the easy part. The binding constraints are septic capacity, where the unit can physically sit given setbacks, whether the electrical service can carry a second kitchen, and access for construction.
Common questions
Can I build an ADU in Massachusetts?
In almost every case, yes. Since 2 February 2025, a single accessory dwelling unit of up to 900 square feet is allowed by right in any single-family residential zoning district in Massachusetts. No special permit, no public hearing, no neighbour veto. Boston is the significant exception, because it zones under its own enabling act rather than M.G.L. c. 40A.
Is the ADU size limit 900 square feet or half my house?
Whichever is smaller. If your house has 3,000 square feet of gross floor area, half is 1,500, so your cap is 900. If your house has 1,400 square feet, half is 700, and 700 is your cap, not 900. Many published summaries get this backwards and say whichever is greater.
What counts toward gross floor area?
Measurement is taken from wall centrelines. Basements, lofts and intermediate floored tiers count. Crawl spaces, garage parking areas, attics and enclosed porches do not. If your lot has more than one principal dwelling, the largest one sets the limit.
Does my town have to approve it?
A town cannot require a special permit or public hearing for a first protected-use ADU, cannot require owner occupancy of either unit, cannot impose a minimum lot size, and cannot cap how many exist. It can still apply site plan review, but the standards have to be clear and objective rather than discretionary.
Do I need to add parking?
A municipality may require at most one additional space, and none at all if any part of the lot is within half a mile of a transit station.
What actually stops most ADU projects?
Not zoning. Wastewater. On an unsewered lot, Title 5 allocates 110 gallons per day per bedroom, and adding bedrooms beyond your system's design flow means upgrading the septic system, which can cost more than the unit itself.
Checking your own lot
We built a free six-question feasibility check that returns your actual size cap, the Title 5 bedroom math for your lot, whether the transit parking exception applies, and a 2026 cost range. It takes about a minute and does not ask for your contact details.
For the full breakdown of the law, including costs across 14 Greater Boston towns, see our guide to accessory dwelling units in Massachusetts.
Sources and limitations
This article reflects 760 CMR 71.00, published 31 January 2025 and effective 2 February 2025, and the Affordable Homes Act, Chapter 150 of the Acts of 2024. Municipal bylaws vary in what they add on top of the state framework, and the regulation has been in force for under two years, so local interpretation is still settling. Confirm specifics with your town's building department before you commit to a design. This is not legal advice.