Massachusetts ADU size rule
How big an ADU does
your house allow?
Massachusetts caps a protected-use Accessory Dwelling Unit at the smaller of 900 square feet or one half of your home’s gross floor area. The second half of that sentence is the one that catches people. Put your numbers in and see the arithmetic.
sq ft
sq ft
Updates as you type.
Largest protected-use ADU
Enter your above-grade floor area and answer the basement question. The number appears here, with the arithmetic behind it.
0sq ft
How that number is reached
- Finished area above grade—
- Basement—
- Your gross floor area—
- Half of that—
- Massachusetts ceiling900 sq ft
- Your maximum — the smaller of the two—
This is an estimate from the numbers you entered, not a determination. Gross floor area is measured from the interior face of exterior walls, and how a specific basement, walkout or half-story is treated depends on the measured condition and on your town’s adopted bylaw. A feasibility study measures it rather than estimating it, and puts the calculation on the sheet. Local bylaws can also permit a larger unit by special permit than the protected-use cap allows by right.
The part that catches people
Basements count. Garages don’t.
Gross floor area is not the number on your tax card and it is not the number on Zillow. It includes basement area at compliant ceiling height whether the basement is finished or not, which routinely makes a house count as larger than the owner expects — and a larger house means a larger allowed ADU. It cuts the other way too: a low basement drops out entirely, and a house that looked big enough suddenly isn’t.
- ✓
Finished floors above grade
First floor, second floor, finished half-stories with headroom. - ×
Crawl spaces
Excluded regardless of area. - ✓
Basements at compliant ceiling height
Finished or unfinished. This is the one people miss. - ×
Garages
Attached or detached, excluded. - ✓
Measured to interior faces
Not exterior footprint, which is what most listings quote. - ×
Unfinished attics
Excluded.
Why the cap matters more than it sounds
Across the studies we have completed, five properties landed within a hair of the ceiling or over it. Four of those five turned on a basement question. A few square feet decides whether a unit is protected by right or needs relief.
Over the line is not the end
A unit larger than the protected-use cap is not automatically prohibited — it just loses by-right protection and goes to your town under its own bylaw. That is a longer road with a real chance of a no.
The number is the easy part
Size is one of six things that decide whether an ADU works. Setbacks, wetlands, access, grading and above all whether your septic or sewer can carry a second dwelling matter at least as much, and cost more when they go wrong.
Get the real number, measured, at no cost
A licensed Massachusetts builder walks the property, measures the gross floor area properly, reads your town’s adopted bylaw against your parcel, and tells you what it would take. Your cost is $0.
More on ADUs: what we build · what an ADU costs in Southeastern Massachusetts · the free feasibility study.