Massachusetts ADU Law: How to Build a By-Right ADU
By the PermitSet drafting team · Updated · 6 min read
Quick answer
Since February 2, 2025, Massachusetts law lets homeowners build one accessory dwelling unit (ADU) by right on a lot in a single-family zoning district — no special permit — as long as it is no larger than 900 square feet or half the floor area of the main house, whichever is smaller. Towns cannot require the owner to live on the property. In the law’s first year, cities and towns reported 1,639 ADU building-permit applications and 1,224 ADUs permitted or approved.
What the law allows
- One “protected use” ADU per lot, by right, in single-family residential zones.
- Size: up to 900 sq ft or 50% of the principal dwelling’s gross floor area, whichever is smaller.
- Detached, attached, or within the existing house (for example a basement or garage conversion).
- No special permit and no owner-occupancy requirement may be imposed for a protected use ADU.
What your town can still require
Towns may still apply reasonable dimensional rules (such as setbacks and height that apply to the main house), site plan review, and the state building and fire codes. State rules also limit how much parking a town can require. Boston is not covered by this statute and has its own ADU program, so Boston homeowners should follow the city’s rules.
Drawings you’ll need for the building permit
- Site plan showing the ADU location, setbacks, and lot coverage.
- Floor plans, elevations, and sections of the ADU (and of the existing house if it’s an attached or interior unit).
- Foundation and framing plans, or engineer’s drawings where required.
- Egress, fire separation, and smoke/CO alarm details.
- Energy-code documentation under the Massachusetts energy code your town has adopted (base or stretch code).
Typical timeline
Permit-ready ADU drawings typically take 5–10 business days to prepare once measurements or a survey are available. Local plan review then varies by town. Submitting a complete package matched to your town’s checklist is the best way to avoid correction cycles.
Frequently asked questions
Do I need a special permit to build an ADU in Massachusetts?
No. Since February 2, 2025, one ADU up to 900 sq ft (or half the main house’s floor area, if smaller) is allowed by right in single-family zones. Towns may require site plan review and building permits.
Do I have to live on the property to build an ADU in Massachusetts?
No. Towns cannot impose owner-occupancy requirements on protected use ADUs.
Does the Massachusetts ADU law apply in Boston?
No. Boston has its own ADU rules and program. Boston permitted the most ADUs of any Massachusetts municipality in 2025.
Building an ADU in Massachusetts?
We prepare permit-ready ADU drawings matched to your town’s requirements. Fixed quote within 1 business day.
Get a Fixed Quote →Sources
- Mass.gov — Accessory Dwelling Units
- Mass.gov — Governor Healey announces more than 1,200 ADUs approved in first year
- Mass.gov — Protected use ADU final regulations (760 CMR 71.00)
This guide is general information, not legal advice. Building codes and local ordinances change — always confirm requirements with your local building department.