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

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?

A garage conversion in Boston typically costs $100,000-$220,000 — well below the $200,000-$400,000 cost of a new detached ADU, because the foundation, walls, and roof already exist. Budget separately for permit fees ($4,500-$7,500) and any electrical or utility upgrades the conversion triggers.

Do I need a permit to convert a garage in Boston?

Yes. Converting a garage to a living unit in Boston requires building permits even if the exterior stays the same — the work has to meet code for insulation, egress, ventilation, electrical, and a kitchen/bathroom. The ADU permit timeline is 90-150 days, and the process is: Interior ADU: building permit through Inspectional Services (no zoning relief). Detached/attached: building permit + ZBA variance (except Mattapan)

Do I have to replace the parking if I convert my garage in Boston?

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.

How long does a garage conversion take in Boston?

Plan on 90-150 days for permit review (4-6 weeks for plan check), then construction. A garage conversion is usually faster to build than a ground-up ADU because the structure already exists.

Can I convert an attached garage into a JADU in Boston?

If the garage is attached to the house, yes — 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, can be carved out of the existing footprint, which includes an attached garage. JADUs are cheaper but may share sanitation with the main home.

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.

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