Since February 2025 a “protected use” accessory dwelling unit is allowed by right in every district that permits single-family dwellings in Massachusetts, Boston excepted — up to 900 square feet, or half the gross floor area of the principal dwelling if that is smaller. That is a real change. It is also widely misunderstood, and the misunderstanding is expensive.
What the state law actually says
The Affordable Homes Act amended M.G.L. c.40A §3 to make a “protected use” accessory dwelling unit permitted by right. In plain terms, for a conforming ADU your town may not require a special permit, may not require you to live on the property, and may not demand more than one additional parking space.
| Rule | What it means | |
|---|---|---|
| 01 | Maximum size | 900 sq ft, or half the gross floor area of the principal dwelling, whichever is smaller |
| 02 | Where | Any district that allows single-family dwellings by right |
| 03 | Permit type | By right. No special permit, no variance |
| 04 | Owner occupancy | Cannot be required |
| 05 | Parking | One additional space maximum, and none within half a mile of transit |
| 06 | How many | One protected ADU per single-family lot |
M.G.L. c.40A §3, as amended by the Affordable Homes Act of 2024, effective 2 February 2025. Verify against the current text before relying on it.
What your town can still require
By right does not mean without process. Your town keeps real authority over how the building is designed, sited and serviced, and that is where projects stall.
- Site plan review — many towns require it for any detached structure over a threshold size. It is not a veto, but it is a hearing and a calendar.
- Dimensional requirements — setbacks, height, lot coverage and separation between structures all still apply, and they vary by district.
- Septic capacity — on an unsewered lot, Title 5 governs. An extra bedroom can mean a new system, and that is often the number that kills the project.
- Building and fire code — 780 CMR applies in full: egress, separation, sprinklers in some cases.
- Overlay districts — wetlands, historic, flood and water supply protection districts each add their own filing.
The three things that most often stop an ADU
| 01 | Septic. The lot is unsewered and the existing system has no reserve capacity. Design around the bedroom count, or budget for an upgrade before you draw anything. | |
| 02 | Separation and setbacks. The only sensible spot for a detached unit sits inside a side setback, or closer to the house than the code allows. A variance is a different project with a different timeline. | |
| 03 | Access. Driveway width, turning radius, or a fire department road requirement that nobody mentioned until the plan review. |
None of these are visible from the street, and all three are knowable before you spend money on drawings. That is what the free check is for.
Find out about your property
Six questions, no cost, and a written summary within one business day. It tells you what state law allows for what you are planning, what your town layers on top with the bylaw section cited, the permitting sequence, a realistic timeline, and the three things that most often go wrong where you live.
In the first six months of 2025, homeowners in 170 Massachusetts communities filed 844 ADU applications. Source: Massachusetts Executive Office of Housing and Livable Communities.