Are backyard cottages allowed in Somerville?
Yes. Somerville's 2019 zoning introduced backyard cottages with footprints just under 600 sq ft at one and a half stories, as the Boston Globe described in October 2026. Since February 2025, the state law has also applied, allowing an ADU of up to 900 sq ft of living space by right in single-family zones.
Is anything about to change?
Possibly. In October 2026 the Globe reported that Mayor Jake Wilson and other officials want to reduce the maximum size allowed for Somerville's backyard cottages, after critics pointed to large cottages sold as condominiums. The same report noted state guidance that the ADU law applies to triple-deckers. Because rules may move, we check the current ordinance on the day we design.
What makes Somerville lots hard?
- Space. Small rear yards and close neighbors leave little room once setbacks are applied.
- Access. Getting materials and equipment past a house built close to its neighbors.
- Utilities. Older water, sewer and electrical service that may need upgrading for a second unit.
Do I need parking?
Often not. Much of Somerville is within half a mile of a Red Line or Green Line station, and as of October 2026 the state bars a parking requirement for an ADU within that distance of a qualifying station. See the parking guide.
Cottage, basement or attic?
It depends on the yard. On many Somerville lots an interior unit or a compact cottage is the realistic choice, because the yard cannot hold more after setbacks. If a tall attic or a dry basement is already there, converting it may be faster and cheaper than building out back.
Related
As of October 2026. Sources: Boston Globe, "Somerville three-decker backyard housing" (October 18, 2026); Mass.gov ADU guidance; MBTA station list.
Check your Somerville lot
Send your address. We check the current cottage rules, setbacks and access for your lot.