A garage conversion is usually the lowest-cost path to an ADU in Somerville — $100,000-$250,000, versus $220,000-$400,000 for a new detached build — because the shell already exists.
Garage Conversion Cost in Somerville
Converting an existing garage in Somerville runs $100,000-$250,000. That's well under the $220,000-$400,000 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 Not published online. ISD charges a plan-review fee at application, credited toward the building permit fee if no plan review is needed; the building permit fee itself is set on the Estimated Cost Affidavit — confirm the current rate with ISD before budgeting.. A converted unit rents for about $2,900-$3,150/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: apply for a building permit with the Inspectional Services Department (ISD) Building Division on the Citizenserve portal (somervillema.gov/citizenserve); no special permit, site plan approval or hearing for a Backyard Cottage or a state Protected Use ADU. An owner living in a 1- or 2-family house may pull the permit; 3-family or larger needs a licensed contractor (CSL, plus HIC for owner-occupied 1-4 unit buildings). Submit an Estimated Cost Affidavit, plans, Workplace Safety and Wage Theft affidavits (contractors also insurance, signed contract, Site Safety Plan, OSHA card). Zoning questions: isdzoning@somervillema.gov; ISD 617-625-6600 x5600, 1 Franey Rd. The review timeline is Not published. ISD posts no review-time target; a plan-review fee is paid when you apply and full plan review precedes issuance — ask ISD (617-625-6600 x5600) for the current queue., with plan check: Full architectural drawings and a site plan showing the 3 ft ambulance-cot walkway, buried utility route and the sprinkler design where required (SZO §3.1.12.f); Fire Prevention Bureau sign-off on the sprinkler exemption for corner, driveway or alley lots.. Pre-approved plans: None. Somerville offers no pre-approved or pre-reviewed ADU plan library; the only related Council action is a Jan 2026 order to discuss an incentive pilot (item 26-0100)..
- Confirm eligibility — ADUs are allowed in Backyard Cottage by right in the Neighborhood Residence (NR) district as the only permitted accessory building type (SZO §3.1.6.c, §3.1.12; Ord. 2023-23); the Urban Residence (UR) district takes the NR building types — confirm cottage eligibility on a UR lot with ISD Zoning. Separately, a state Protected Use ADU is by right on any lot in a district where single-family homes are allowed (NR and UR), including lots with a two- or three-family house (760 CMR 71.02, 71.03(1))..
- Prepare conversion plans — layout, egress windows, insulation, ventilation, smoke/CO detectors, plus a kitchen and bathroom.
- Submit for plan check — Full architectural drawings and a site plan showing the 3 ft ambulance-cot walkway, buried utility route and the sprinkler design where required (SZO §3.1.12.f); Fire Prevention Bureau sign-off on the sprinkler exemption for corner, driveway or alley lots..
- Pull permits and build — fees run Not published online. ISD charges a plan-review fee at application, credited toward the building permit fee if no plan review is needed; the building permit fee itself is set on the Estimated Cost Affidavit — confirm the current rate with ISD before budgeting.; then framing, electrical, plumbing, and final inspection.
Parking Rules for a Garage Conversion
None. Somerville requires no motor-vehicle parking for residential uses in the NR district and none for accessory uses (SZO §3.1.18.a.iii and Table 3.1.18, Ord. 2024-15); state law also bars any ADU parking requirement within 0.5 mile of a transit station, defined to include any MBTA bus stop (760 CMR 71.02, 71.03(2)(b)). Garage conversions sometimes trigger different parking rules than new detached ADUs, so confirm replacement-parking requirements with Somerville before you start. Owner-occupancy is not required, so you can rent the converted unit out.
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 or the Somerville ordinance; a small unit carved from the house is simply an ADU under the state 900 sq ft / 50% cap. 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.
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Somerville Garage Conversion FAQ
How much does a garage conversion cost in Somerville?
Do I need a permit to convert a garage in Somerville?
Do I have to replace the parking if I convert my garage in Somerville?
How long does a garage conversion take in Somerville?
Can I convert an attached garage into a JADU in Somerville?
Compare your options: see the full Somerville ADU rules, estimate your build with the Somerville ADU cost calculator, or read the general garage conversion guide.