Internal, attached, detached, or qualifying modular unit
All ADU guides Massachusetts ADU code path
Can you build an ADU in Massachusetts?
Massachusetts protects one qualifying ADU by right outside Boston; local dimensional review, Title 5 or sewer, building code, wetlands, flood, and historic rules still apply.
Protected use statewide outside BostonState posture
One protected ADU; additional units follow a different local pathProtected or locally permitted count
Property briefs livePaid cited review is available
The state-wide starting point.
These fields stay separate because a broad state policy, a limited geographic mandate, and a property-specific approval are not the same answer.
One protected ADU; additional units follow a different local path
Smaller of 900 square feet or half the principal dwelling's gross floor area
At most one added space; none within 0.5 miles of qualifying transit
Not permitted for the protected ADU
Long-term rental protected; qualifying short-term rentals may be restricted
What the headline does not settle.
These are the common edges that can move a project outside the protected or simplest path.
State ADU protection does not waive building, fire, health, environmental, utility, or private-title requirements.
Local objective standards still matter and may differ among municipalities.
A second unit, larger unit, short-term rental, or separate sale may follow a different rule than the protected baseline.
What decides the actual lot.
State law is one layer. These checks turn it into a parcel-specific feasibility answer.
- Zoning district, legal principal dwelling, and the municipality's current ADU ordinance
- Lot coverage, setbacks, height, access, parking, and any nonconforming structures
- Sewer or septic capacity, water service, utilities, and fire access
- Wetlands, flood hazard, coastal, wildfire, historic, HOA, deed, and condominium restrictions
A practical verification path.
Stop at the first unresolved layer rather than designing past a constraint.
Confirm the parcel, governing municipality, zoning district, and legal use of the primary home.
Choose an internal, attached, or detached concept and test it against the controlling size and dimensional rules.
Retrieve the current zoning text, permit checklist, utility records, and every overlay that applies to the site.
Resolve wastewater, building, fire, access, environmental, and private-title constraints before final design.
Submit the required plans, complete inspections, and obtain every approval and occupancy document before use.
Sources behind this guide.
Official sources support published legal claims. Comparison sources are labeled and used for orientation only.
Reviewed August 6, 2026.
State rules are the start, not the verdict.
ZoningTrace can build the cited property-specific rule stack for this jurisdiction.
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