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Basement ADU on the North Shore

Many North Shore homes have a full basement that could become an ADU. To be a legal unit it has to be dry, have proper egress and ceiling height, and meet fire separation rules. We connect you with independent local builders who can tell you whether yours qualifies and quote the work.

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Finished basement apartment with a kitchenette and egress window

Why the 2025 ADU law matters

The rules changed statewide. Here is what that means for a North Shore lot.

Allowed by right

Since February 2, 2025, one ADU per lot is allowed by right in single-family zoning districts. Towns cannot require a special permit for a protected ADU.(Mass.gov: Accessory dwelling units (ADUs))

No owner-occupancy or family requirement

Towns cannot require the owner to live on the property or require that the ADU be occupied by family.(Mass.gov: Accessory dwelling units (ADUs), 760 CMR 71.00: Protected use accessory dwelling units)

Size and entrance rules

A protected ADU can be no larger than half the main house's gross floor area or 900 square feet, whichever is smaller, and must have its own entrance.(760 CMR 71.00: Protected use accessory dwelling units)

Local rules still apply

Towns may still use site plan review, setbacks and dimensional rules, short-term rental limits, and Title 5 septic requirements.(Mass.gov: Accessory dwelling units (ADUs), MassDEP: Guidance on Title 5 compliance for accessory dwelling units (PDF))

From idea to move-in

  1. Moisture first. Any water or humidity problem is fixed before finishing. Living space over an unresolved leak is a mold problem waiting to happen.
  2. Egress and separation. Plans for a code-compliant entrance and emergency escape openings, and fire separation from the main house.
  3. Permits. Building, electrical, plumbing and, where needed, gas permits.
  4. Finish and inspect. Framing, insulation, kitchen and bath, then final inspections.

Is this a good fit?

  • You have a full basement with decent ceiling height
  • A walk-out or a spot where a code-compliant egress can be added
  • The basement stays dry, or you are willing to fix water problems first
  • You want an ADU without changing the outside of the house much
  • You want to keep the yard

Further reading: Mass.gov: Accessory dwelling units (ADUs) · 760 CMR 71.00: Protected use accessory dwelling units · MassDEP: Guidance on Title 5 compliance for accessory dwelling units (PDF) · Mass.gov: Check a home improvement contractor registration · Mass.gov: When is a construction supervisor license required?

Every job is different. The pro you speak with will explain their own approach and pricing. North Shore ADU Pros does not perform, supervise, or guarantee any work.

ADU questions

Does a basement ADU count toward the size cap?

Under state rules a protected ADU can be no larger than half the gross floor area of the main house or 900 square feet, whichever is smaller.

What about radon?

Basements are where radon concentrates. The EPA recommends testing; ask your builder about testing before and after.

My basement gets water. Can it still be an ADU?

Only after the water problem is solved. Ask for waterproofing to be priced as part of the project.

How many ADUs have been approved in Massachusetts?

The state's Executive Office of Housing and Livable Communities reported 2,084 ADUs approved and 2,824 applications statewide from 2025 through the first half of 2026.

How much parking can a town require?

No more than one space for a protected ADU, and none if the lot is within half a mile of a commuter rail, subway, ferry or bus station.

Other ADU types

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