Frost decides how a Boston garage foundation is built. The Massachusetts residential code sets the frost line at 48 inches, and a garage that does not reach it will move every winter. HomeGuide puts a two-car garage slab at $2,400 to $6,900 nationally.
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 slab cost?
A concrete slab costs $6 to $12 per square foot installed by HomeGuide's slab page, or $2,400 to $6,900 for a two-car garage floor of 400 to 576 square feet. A 20x20 slab runs $2,400 to $4,800. Those figures are for the slab, not the frost-depth footings or walls a Massachusetts garage needs under it.
How thick should a garage slab be?
Most two-car garage slabs are 4 to 6 inches of reinforced concrete, which is common practice as HomeGuide describes it, not a code figure. HomeGuide lists 4 inch slabs at $6 to $8 per square foot and 6 inch slabs at $8 to $10. We use the thicker end for heavy trucks, a lift or a workshop. Your plans and the code set the minimum.
Do garage footings need to go below the frost line in Massachusetts?
Yes. Table R301.2(1) of the Massachusetts residential code sets the frost line depth at 48 inches, so garage footings or frost walls go down that far. The code also allows frost-protected shallow foundations, which use insulation to keep frost out, but they have to be designed for it. A slab simply poured on the ground with no footing is what heaves and cracks in Boston winters.
Can you build a garage on a sloped lot?
Yes. On a sloped lot the foundation steps down the hill, with taller walls on the low side, and the high side may need a retaining wall. The driveway grade into the garage matters too: it should not send water to the door. Hills in Brookline, Medford, Malden and Waltham make this a common job.
What if there is ledge under my garage site?
Ledge is common around Boston: puddingstone in parts of the city, granite in Quincy and near the Blue Hills, and rock near the Fells. Footings that bear on sound rock can stop at the rock, but the ledge may still have to be cut or broken to set the slab level. HomeGuide publishes no garage figure for rock work, so ask for a written rock clause in the quote.
How do I keep water from running into my garage?
Keep water out of a garage with grade, slope and gutters. The ground outside should fall away from the walls and the door. The slab should slope gently toward the door or a drain so meltwater from cars runs out. Gutters and downspouts carry roof water away. Where the driveway runs downhill to the door, a trench drain across the opening catches it.
Does a detached garage need its own foundation?
Yes. A detached garage needs its own foundation, with footings or frost walls down to the 48 inch frost depth and a slab inside. An attached garage needs the same depth so it does not move apart from the house. A garage set only on an old patio or driveway slab will crack and shift.
How much does it cost to demolish an old garage?
Garage demolition costs $3 to $10 per square foot, or $1,000 to $5,000 on average, by HomeGuide's demolition page. A wood one-car garage runs $1,000 to $2,500 and a brick two-car $2,000 to $5,000. Taking out the old concrete floor adds $1 to $3 per square foot, and HomeGuide lists a dumpster at $300 to $700 per week.
How long does a slab cure before framing?
Framing can usually start a few days after the pour, once the concrete has set enough to work on, but curing goes on for weeks. Concrete is commonly tested for its design strength at 28 days, so heavy loads wait. In Boston's cold months the slab must be kept from freezing while it cures, with blankets or heat, and curing is slower.
Do I need to call Dig Safe before the excavation?
Yes. Massachusetts requires notice to Dig Safe at least 72 hours ahead, not counting weekends and legal holidays, with the dig area premarked in white. Private lines to an old garage, such as a buried power cable, are not always marked, so we look for them too.
What goes under a good garage slab?
A good garage slab sits on firm, compacted soil and a layer of gravel. HomeGuide lists a gravel base at $1 to $3 per square foot. A vapor barrier under the slab keeps ground moisture down, and it matters if the garage may become living space later. HomeGuide publishes no figure for the barrier or for rebar.
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.