Garage Conversion in Somerville, MA: Cost, Permits & Rules (2026)

A garage conversion is usually the lowest-cost path to an ADU in Somerville — $150,000-$250,000, versus $275,000-$450,000 for a new detached build — because the shell already exists.

Garage Conversion Cost in Somerville

Converting an existing garage in Somerville runs $150,000-$250,000. That's well under the $275,000-$450,000 typical cost of a ground-up detached ADU, because you're reusing the foundation, walls, and roof. The biggest variables are whether the slab and framing are sound, what utility/electrical upgrades are needed, and your finish level. Permit fees add Confirm with Inspectional Services — building permit fees are valuation-based; budget several thousand dollars on a typical $300K+ build. A converted unit rents for about $2,600-$3,000/mo (1BR) in Somerville.

The Somerville Garage Conversion Permit Process

A garage-to-ADU conversion in Somerville needs building permits even if the exterior is unchanged. Approval process: By right: building permit through Inspectional Services (apply online via Citizenserve at somervillema.gov/citizenserve); no special permit, site plan review, or discretionary zoning approval since the 2022 amendment and the 2025 state law. Design standards apply: 3-ft paved walkway sized for an ambulance cot, buried utility services, and automatic sprinklers unless the cottage is on a corner lot, served by a driveway, or abuts an alley The review timeline is Building-permit plan review typically 4-8 weeks — Confirm with Inspectional Services (617-625-6600 ext. 5600, isd@somervillema.gov), with plan check: Full architectural drawings plus site plan; sprinkler plan required for most mid-block cottages. Pre-approved plans: None — Somerville does not offer a pre-approved ADU plan library.

1. Confirm eligibility — ADUs are allowed in By right in Neighborhood Residence (NR) district as the 'Backyard Cottage' accessory building type (SZO §3.1.12); state law (760 CMR 71) additionally protects one ADU by right on lots with a single-family dwelling in any district where single-family homes are permitted.
2. Prepare conversion plans — layout, egress windows, insulation, ventilation, smoke/CO detectors, plus a kitchen and bathroom.
3. Submit for plan check — Full architectural drawings plus site plan; sprinkler plan required for most mid-block cottages.
4. Pull permits and build — fees run Confirm with Inspectional Services — building permit fees are valuation-based; budget several thousand dollars on a typical $300K+ build; then framing, electrical, plumbing, and final inspection.

Parking Rules for a Garage Conversion

None required for the ADU — state law bars parking requirements within 1/2 mile of a subway/rail/bus station, which covers virtually all of Somerville (Davis Red Line + 5 Green Line Extension stations) Garage conversions sometimes trigger different parking rules than new detached ADUs, so confirm replacement-parking requirements with Somerville before you start. Owner-occupancy: Not allowed — prohibited statewide by the Affordable Homes Act effective Feb 2, 2025.

The JADU Option for an Attached Garage

If your garage is attached to the house, you can convert it into a junior ADU (JADU) — capped at No JADU category in Massachusetts law and built within the existing walls. JADUs are cheaper and faster to permit than a full ADU, though they may share sanitation with the main home. A full ADU gives you more size and a separate unit; a JADU is the budget route.

Somerville Garage Conversion FAQ