What is a detached ADU?
It is a self-contained home on the same lot as your house but in a separate building. In Massachusetts law an ADU has its own sleeping, cooking and bathroom facilities and its own entrance. A detached unit gives both households the most privacy, and it is the type most people picture when they say "backyard cottage."
Where can I build one?
As of October 2026, the Affordable Homes Act lets owners in single-family zoning districts build one ADU by right in cities and towns governed by the state Zoning Act, which includes Quincy, Newton, Somerville and Brookline. The City of Boston is outside that law. Boston allows detached ADUs by right on owner-occupied residential lots in Mattapan, and a July 2026 draft would extend them to parts of Hyde Park, Roslindale and West Roxbury. See Boston's ADU rules.
How big can a detached ADU be?
Under state law, as of October 2026, the cap is 900 sq ft or half the gross floor area of the main home, whichever is smaller. A town can be more generous, and some are:
- Newton allows a detached ADU of up to half the main building or 1,000 sq ft by right, up to 1,200 sq ft in some cases, and up to 1,500 sq ft by special permit, under amendments passed in April 2025.
- Brookline holds a new detached ADU's footprint to 600 sq ft in many districts and 700 sq ft in its S-10 and SC-10 districts, under its May 2026 bylaw update.
- Somerville has allowed backyard cottages since 2019, with footprints just under 600 sq ft at one and a half stories.
How close to the lot line can it sit, and how tall can it be?
The state law lets towns keep reasonable setback, bulk and height rules, so the answer is local. Two examples, as of October 2026: Newton allows up to 2.5 stories when the ADU meets the setbacks of a new house, or 1.5 stories and 22 feet with 5-foot setbacks. Brookline caps a new or expanded detached ADU at 22 feet and generally requires the house's side setback and half its rear setback.
Do I need to add a parking space?
Usually no more than one, and often none. As of October 2026, a town covered by the state law may require at most one space, and none if the lot is within half a mile of a subway, commuter rail, ferry or bus station. Read our parking guide.
What does a new cottage need that a conversion does not?
- A frost-protected foundation and a full new structure and roof.
- Water, sewer and electrical runs from the street or the house, which can be long on a deep lot.
- Room to get materials and equipment into the backyard.
- Energy code compliance for new construction. All-electric designs can qualify for Mass Save incentives.
When is a detached ADU the wrong choice?
It depends on the lot. If setbacks leave a sliver of buildable yard, or utilities would have to cross a long driveway, an addition or an interior unit can give you the same square footage for less. We sketch the options side by side before you commit to one.
What does a detached ADU cost?
As of October 2026, the Massachusetts Clean Energy Center puts a studio detached ADU at about $200,000 to $300,000 and a two-bedroom at about $250,000 to $450,000, including site work. Your quote will be written and itemized. See the full cost guide.
Related
Rules as of October 2026. Sources: Mass.gov ADU guidance; City of Newton, April 2025 ADU amendments as reported by Fig City News; Town of Brookline, Article 14 vote (May 2026); Boston Globe, October 2026; Boston Planning Department; MassCEC ADU budgeting guide.
See if a cottage fits your yard
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