Attached ADU additions near Boston

What is an attached ADU?

It is a new unit built onto the side or back of your house, or over an attached garage. It shares structure with the main home, so it usually needs less site work than a separate cottage, but it must be fully separated from your living space for fire safety and sound.

How must the entrance work?

As of October 2026, Mass.gov guidance says an ADU needs a separate entrance, either straight from the outside or through a shared hall or corridor that meets the state building code's egress rules. On an addition we usually give the unit its own exterior door, which is simpler for tenants and for inspections.

How large can the addition be?

The state cap applies: 900 sq ft or half the gross floor area of the main home, whichever is smaller, as of October 2026. On a modest Cape or ranch, the half-the-house rule is often the limit that binds, so we measure the existing floor area carefully before sizing the plan. Local setbacks, height limits and floor area rules still apply to the addition.

Can I build an attached ADU in the City of Boston?

Not through the citywide ADU path, as of October 2026. Boston's current allowance is for units inside an existing home without exterior alterations, so an addition generally needs zoning relief. The city's 2026 draft for Hyde Park, Roslindale and West Roxbury focuses on detached units. See Boston's rules.

What does an addition involve?

• New foundation and framing tied into the existing house.
• A fire-rated wall, and ceiling where units stack, between the two homes.
• Separate heating and cooling, and usually a separate electrical panel.
• Roof and siding detailing so the addition reads as part of the house.
• Historic commission review if the property is in a local historic district.

Related

Rules as of October 2026, from Mass.gov ADU guidance, the Boston Planning Department and the MassCEC ADU budgeting guide.