Can I Build an ADU on My Property in Massachusetts?
Short answer: in most cases, yes. Since February 2, 2025, Massachusetts law lets homeowners build one accessory dwelling unit (ADU) by right in single-family zoning districts. That means no special permit or variance, only a regular building permit.
The key rules (Affordable Homes Act)
- Size: up to 900 sq ft, or half the size of your main house, whichever is smaller.
- Where: single-family residential zoning districts statewide.
- Owner-occupancy: towns cannot require you to live on the property to build or rent the ADU.
- Parking: towns can require at most one parking space, and none if you are within 0.5 miles of public transit.
- Type: it can be attached, detached, or inside the existing house (basement, garage conversion).
What your town can still control
Towns can still apply reasonable rules on setbacks, height, lot coverage and design, as long as they are not stricter than for a single-family home. Towns can also limit short-term rentals. Boston is not covered by the state by-right rule and has its own ADU process.
Steps to build an ADU in Massachusetts
- Check your zoning district and lot setbacks with your town.
- Decide the ADU type and size (max 900 sq ft).
- Get drawings and a written estimate.
- Apply for the building permit.
- Build, inspect and get your certificate of occupancy.
FAQ
Do I need a special permit for an ADU in Massachusetts?
Not for one ADU in a single-family zone. It is allowed by right; you only need a building permit.
Can I rent out my ADU?
Yes. The state law bans owner-occupancy requirements, but your town may restrict short-term rentals.
How big can an ADU be in Massachusetts?
Up to 900 sq ft or half of your main house's floor area, whichever is smaller.
Does the law apply in Lowell?
Yes, the by-right rule applies in single-family zones in Lowell and nearby towns. Boston is the main exception.
This page is general information, not legal advice. Always confirm with your local building department.
Related services and guides
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