A garage conversion is usually the lowest-cost path to an ADU in Boston — $100,000-$220,000, versus $200,000-$400,000 for a new detached build — because the shell already exists.
Garage Conversion Cost in Boston
Converting an existing garage in Boston runs $100,000-$220,000. That's well under the $200,000-$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 $4,500-$7,500. A converted unit rents for about $1,800-$2,400/mo (1BR) in Boston.
The Boston Garage Conversion Permit Process
A garage-to-ADU conversion in Boston needs building permits even if the exterior is unchanged. Approval process: Interior ADU: building permit through Inspectional Services (no zoning relief). Detached/attached: building permit + ZBA variance (except Mattapan) The review timeline is 90-150 days, with plan check: 4-6 weeks. Pre-approved plans: Not available.
- Confirm eligibility — ADUs are allowed in Interior ADUs as-of-right in owner-occupied 1-3 family homes citywide; detached/attached by ZBA variance (by-right only in Mattapan).
- Prepare conversion plans — layout, egress windows, insulation, ventilation, smoke/CO detectors, plus a kitchen and bathroom.
- Submit for plan check — 4-6 weeks.
- Pull permits and build — fees run $4,500-$7,500; then framing, electrical, plumbing, and final inspection.
Parking Rules for a Garage Conversion
None required for an interior ADU Garage conversions sometimes trigger different parking rules than new detached ADUs, so confirm replacement-parking requirements with Boston before you start. Note: Required (owner-occupied 1-3 family home; condos and LLCs ineligible).
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 Interior ADU (the as-of-right type): up to 900 sq ft and no more than 50% of living area, within existing footprint 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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Boston Garage Conversion FAQ
How much does a garage conversion cost in Boston?
Do I need a permit to convert a garage in Boston?
Do I have to replace the parking if I convert my garage in Boston?
How long does a garage conversion take in Boston?
Can I convert an attached garage into a JADU in Boston?
Compare your options: see the full Boston ADU rules, estimate your build with the Boston ADU cost calculator, or read the general garage conversion guide.