Massachusetts changed the rules in 2024: an accessory dwelling unit of up to 900 square feet is now allowed by right in single-family zoning districts. That makes a garage with an apartment above, or a garage turned into a unit, a real option on many Greater Boston lots.
Every dollar figure on this page is a national figure published by HomeGuide, not a quote from us. HomeGuide publishes no Boston or Massachusetts garage prices. Lot access, ledge, frost depth and snow load all move what a real job here costs. Figures read in October 2026.
How much does a garage with an apartment above cost?
A garage with an apartment above costs $200 to $350 per square foot, or $80,000 to $300,000, by HomeGuide's garage apartment page. A prefab version runs $60 to $100 per square foot.
| Garage footprint | HomeGuide national range |
|---|---|
| 20 x 20 (400 sq ft) | $80,000 to $140,000 |
| 20 x 24 (480 sq ft) | $96,000 to $168,000 |
| 24 x 24 (576 sq ft) | $115,000 to $200,000 |
| 36 x 24 (864 sq ft) | $173,000 to $300,000 |
Can I build an ADU over or in my garage in Massachusetts?
Yes, in most single-family zoning districts. Section 8 of the Affordable Homes Act (Chapter 150 of the Acts of 2024) amended the state Zoning Act so that one accessory dwelling unit is allowed by right from February 2, 2025. The unit can be no larger than half the gross floor area of the main home or 900 square feet, whichever is smaller, and needs its own entrance. The state's rules are in 760 CMR 71.00. Towns can still apply site plan review, setbacks and limits on bulk and height. Boston has its own zoning code, so ask the city how the rules apply at your address.
Can I build a second story over my existing garage?
Often, if the walls and foundation can carry it. HomeGuide puts a second story over a garage at $60,000 to $200,000 or more. Over an attached garage it is usually a bedroom or family room joined to the house; over a detached garage it is a studio, office or ADU.
Can my garage foundation hold a room on top?
Only an engineer can tell you whether a garage foundation can hold a room, after looking at it. Many older Boston garages sit on a thin slab with no real footing, which cannot carry a second floor, so the plan becomes new footings or a full rebuild. A newer garage on frost-depth footings may only need reinforcing. Snow load on the new roof is part of that calculation.
How much does it cost to convert a garage into a bedroom or living space?
Converting a garage costs $5,000 to $50,000 or more, or $50 to $200 per square foot as living space, by HomeGuide's conversion page. A bedroom runs $5,000 to $17,000 and a garage-to-apartment or ADU conversion $20,000 to $100,000 or more. A two-car conversion runs $10,000 to $50,000 or more.
Does converting a garage hurt resale value?
It can around Boston. HomeGuide's conversion page says converting a garage in a cold region or an area with limited parking may devalue the property, and Boston is both. A legal unit that brings rent can offset that, but ask a local agent before you give up the parking.
Do I have to replace or raise the slab when converting?
Not always. A sound slab can stay, but in a living space it needs a vapor barrier, insulation and a new floor on top, often raised to meet the house floor. A cracked, sunken or tipped slab should be replaced. HomeGuide puts garage floor replacement at $10 to $16 or more per square foot, or $4,000 to $9,200 or more for two cars.
Can I rent out an apartment above my garage?
Under the state ADU rules, a town cannot require the owner to live on the property or require the tenant to be family, so a protected ADU can usually be rented. Towns may restrict or ban short-term rentals. Boston and some towns also have local rental rules, so check them before you sign a lease. This is general information, not legal advice.
How much parking does a garage ADU need in Massachusetts?
The state ADU rules say a town cannot require any parking for an ADU within half a mile of a commuter rail station, subway station, ferry terminal or bus station, and no more than one space elsewhere. Much of Greater Boston is near transit, so the garage may be freed for living space without new parking.
What does a garage apartment need that a garage does not?
A garage apartment needs what any home needs: a separate entrance, stairs that meet code, heat, insulation, plumbing, smoke and carbon monoxide alarms, and windows for light and escape. The floor between the cars and the unit also needs fire separation. Plan them all before the frame goes up, since moving a stair later is costly.
Want a number for your own lot? We visit, measure and put the scope and price in a written quote. Call or text (617) 326-7524.