A garage conversion is usually the lowest-cost path to an ADU in Cambridge — $105,000-$230,000, versus $210,000-$420,000 for a new detached build — because the shell already exists.
Garage Conversion Cost in Cambridge
Converting an existing garage in Cambridge runs $105,000-$230,000. That's well under the $210,000-$420,000 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 $3,500-$5,500. A converted unit rents for about $1,900-$2,500/mo (1BR) in Cambridge.
The Cambridge Garage Conversion Permit Process
A garage-to-ADU conversion in Cambridge needs building permits even if the exterior is unchanged. Approval process: Building permit - as-of-right, no discretionary design review The review timeline is 60-90 days, with plan check: 3-4 weeks. Pre-approved plans: Not available.
- Confirm eligibility — ADUs are allowed in All residential districts - Cambridge eliminated single-family-only zoning in Feb 2025 (Residence C-1 citywide); one ADU is by-right under the state law.
- Prepare conversion plans — layout, egress windows, insulation, ventilation, smoke/CO detectors, plus a kitchen and bathroom.
- Submit for plan check — 3-4 weeks.
- Pull permits and build — fees run $3,500-$5,500; then framing, electrical, plumbing, and final inspection.
Parking Rules for a Garage Conversion
None - Cambridge abolished all parking minimums citywide in Oct 2022 (state law would otherwise allow 1 space beyond 0.5 mi of transit) Garage conversions sometimes trigger different parking rules than new detached ADUs, so confirm replacement-parking requirements with Cambridge 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 N/A (Massachusetts has no JADU category) 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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Cambridge Garage Conversion FAQ
How much does a garage conversion cost in Cambridge?
Do I need a permit to convert a garage in Cambridge?
Do I have to replace the parking if I convert my garage in Cambridge?
How long does a garage conversion take in Cambridge?
Can I convert an attached garage into a JADU in Cambridge?
Compare your options: see the full Cambridge ADU rules, estimate your build with the Cambridge ADU cost calculator, or read the general garage conversion guide.