Garage-to-ADU Conversions in Massachusetts: Costs, Rules, and What to Expect

If you have a structurally sound garage, you may be sitting on the cheapest legal rental unit in Massachusetts. Garage conversions skip the most expensive parts of new construction — foundation, framing, roof — which is why they land at $120K–$220K instead of the $250K+ of a ground-up cottage. Here's what the project actually involves.

Is Your Garage a Candidate?

• Slab condition: cracked, sloped slabs (most garage floors pitch toward the door) get a floating insulated floor over them — figure that into ceiling height.
• Ceiling height: you need 7 feet finished for habitable space. Many garages make it; low-slung ones may need structural work.
• Structure: rot, undersized framing, or a failing roof get fixed first — sometimes the honest answer is that a rebuild costs less than a rescue.

What the Conversion Involves

• Envelope: insulated walls and ceiling to current energy code (R-values a garage never had), new windows and doors where the overhead door was.
• Utilities: attached garages tap the house systems; detached ones need trenched water, sewer, and electric — $10K–$25K depending on run length.
• Interior: kitchen, bathroom, heating/cooling (usually heat pump mini-splits), and fire-rated separation where required.

Permits & Timeline

Under the state ADU law, a conforming garage conversion is by-right — standard building permit, no zoning board. Design through certificate of occupancy typically runs 4–6 months. The full sequence is in our ADU permit guide.

The Parking Question

Converting your garage removes parking. Towns may require at most one replacement space — zero near transit — and a driveway usually satisfies it. We confirm this in feasibility before you spend a dollar on design.

Our ADU construction team has converted garages across Medford, Quincy, and Greater Boston — free feasibility reviews if you're wondering about yours.