ADUs are allowed by right
The Affordable Homes Act, signed in August 2024, made accessory dwelling units a by-right use in single-family zoning districts across Massachusetts starting February 2, 2025. For a “protected use” ADU, towns can no longer require a special permit, and they cannot require the owner to live on the property.
How big can an ADU be?
A protected-use ADU can be up to 900 square feet or half of the gross floor area of the principal dwelling, whichever is smaller. It can be inside the house (basement or attic), attached, or in a detached structure such as a converted garage or a new backyard cottage.
What towns can still regulate
Municipalities may apply reasonable regulations — for example dimensional setbacks, site plan review and building design standards — and parking requirements are limited, especially near transit. Rules vary by town, so check the local bylaw or ordinance early.
You still need a building permit
Every ADU must comply with the Massachusetts State Building Code. Typical review points are ceiling height, emergency egress (especially in basements), fire separation between units, smoke and CO alarms, stairs, and ventilation. A clear permit drawing set is what gets these answered on the first review.
Typical ADU drawing set
- Existing conditions plans (measured on site)
- Proposed floor plans with dimensions and egress
- Sections showing ceiling heights and fire separation
- Elevations for exterior changes
- Site plan information for detached ADUs
This guide is general information, not legal advice. Local rules change — confirm with your building department. Ready to start? See our ADU design & permit drawings service.