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Basement and attic ADUs in Boston

In the City of Boston, an apartment inside the house you already own is the most direct way to add a unit today. We turn basements and attics into separate, code-compliant homes. Licensed and insured.

Check my house Boston ADU rules

Finished attic room: Beige 3-seat sofa near the window

Why are interior ADUs the main option in Boston?

Boston is not covered by the state's 2024 ADU law, and its own zoning, as of October 2026, lets owner-occupants create an ADU within existing space such as an unused basement or attic, with no exterior alterations. The Boston Planning Department says detached ADUs are allowed by right only in Mattapan so far. Boston reports that 212 ADUs were permitted in the city between 2019 and July 2025, as cited by the Pioneer Institute.

Can I build a basement or attic ADU outside Boston?

Yes. In Quincy, Newton, Somerville and Brookline, an interior unit is a protected-use ADU under state law when it meets the size cap of 900 sq ft or half the house, whichever is smaller, as of October 2026. Newton's local rules allow larger internal units in some cases.

What does a basement need to become a home?

Headroom. Habitable rooms must meet the building code's minimum ceiling height. Many older Boston basements fall short and need the floor lowered.
A way out. Sleeping rooms need an emergency escape opening, which usually means a larger window and a window well.
Dry walls and floors. Drainage, a sump where needed, and insulation that will not trap moisture.
Separation. Fire-rated construction between the unit and the rest of the house.
Mechanicals. Boilers, water heaters and panels often need to move or be enclosed.

Finished attic room: A bedroom with a bed and a fireplace in it

What does an attic need?

A legal stair, enough headroom over enough of the floor, insulation at the roof line, and a second way out where the code requires it. In Boston, dormers count as exterior changes, so an attic that only works with new dormers may need zoning relief rather than the citywide ADU path.

Is there help paying for it in Boston?

The City of Boston and the Boston Home Center offer ADU financing support and technical assistance, and the city has described a 0% loan for building an approved design. Terms and eligibility change, so we point you to the current program when we quote. Outside Boston, MassHousing runs an ADU loan program for income-eligible borrowers.

When does a basement unit not pencil out?

It depends on the height and the water. A basement that needs underpinning to gain headroom and also takes on water in spring storms can cost far more than its square footage suggests. An attic with a steep roof and no room for a code stair can be the same story. We measure both before recommending either.

What does an interior ADU cost?

As of October 2026, MassCEC estimates an interior conversion at about $120,000 to $270,000. See the cost guide.

Is your basement or attic a candidate?

Send your address and tell us which space you have in mind. We check the zoning path and the headroom questions first.