Updated November 1, 2026. General information only, not legal, engineering, or insurance advice.
What changed
The Affordable Homes Act (Chapter 150 of the Acts of 2024), signed August 6, 2024, made one "protected use" ADU per lot allowed by right in single-family zoning districts. It took effect February 2, 2025. The state regulations are 760 CMR 71.00.
What a town cannot require for a protected ADU
- A special permit or other discretionary approval
- That the owner live on the property
- That the occupants be related to the owner
- More than one parking space, or any parking if the lot is within half a mile of a commuter rail, subway, ferry or bus station
What still applies
- Size: no larger than half the gross floor area of the main house or 900 square feet, whichever is smaller
- A separate entrance meeting building code egress requirements
- Reasonable local rules such as site plan review, setbacks, and height and dimensional limits
- Short-term rental restrictions the town adopts
- Title 5 septic rules where there is no sewer. MassDEP says adding an ADU that increases design flow is treated as new construction.
- The state building code, plus building and trade permits
Next steps
Start with your town's building or planning department and your property records. A designer can then tell you what fits on your lot. See ADU design and build, and check any contractor's HIC registration and construction supervisor license on Mass.gov.
Get a free quote
If you would like a local pro to take a look, use the form on this page. It is free to ask and there is no obligation. Related: ADU Design & Build, Detached ADU (Backyard Cottage), Garage Conversion ADU.
Sources
- Mass.gov: Accessory dwelling units (ADUs)
- 760 CMR 71.00: Protected use accessory dwelling units
- Mass.gov: Accessory dwelling unit (ADU) FAQs
- MassDEP: Guidance on Title 5 compliance for accessory dwelling units (PDF)
- Mass.gov: Check a home improvement contractor registration
- Mass.gov: When is a construction supervisor license required?