Garage slab in Austin: floor and foundation
A garage slab carries the walls and roof as well as the cars, so it isn't a patio with a building on it. A common way to build it is a monolithic slab: a floor with deeper, wider edges poured at the same time, sized on the plans. The floor slopes toward the big door, as the residential code requires for the parking area (IRC R309.1).
A garage with a room above is a slab foundation job; a roof on posts with no walls is a carport slab.
Garage slab sizes and cost in Austin
| Garage | Area | projectcosted Austin ($5.63 to $11.26) | HomeBlue Austin ($6 to $10) |
|---|---|---|---|
| One-car, 12x20 | 240 sq ft | $1,351 to $2,702 | $1,440 to $2,400 |
| One-car roomy, 14x24 | 336 sq ft | $1,892 to $3,783 | $2,016 to $3,360 |
| Two-car, 20x20 | 400 sq ft | $2,252 to $4,503 (published) | $3,400 to $5,900 (published) |
| Two-car, 24x24 | 576 sq ft | $3,243 to $6,486 | $3,456 to $5,760 |
| Three-car, 24x36 | 864 sq ft | $4,864 to $9,729 | $5,184 to $8,640 |
Swipe the table sideways to see every column →
Rows multiply area by each source's Austin per-sq-ft rate except where marked published. Thickened edges, a step at the door and a vapor retarder are extra.
People also search for the cost to pour a concrete slab for a garage; the Austin slab cost page has the line items behind these numbers.
Garage slab details that matter later
Slope and the door
The slab falls toward the main door so wash water, rain blown in under the door and drips leave the garage. The door opening itself can get a small step-down or a recess so water outside can't run back in.
Edges under the walls
Wall loads go into the thickened edge. The code minimums for footings, 12 inches wide, 6 thick and 12 below undisturbed ground, apply to that edge, and on Austin clay the engineer usually goes deeper. More on the concrete footings page.
Vapor retarder or not
IRC R506.2.3 doesn't require a vapor retarder under a garage. If the space might become a workshop with AC or a garage apartment, install the 10-mil retarder now.
Permits and impervious cover
The City of Austin exempts detached accessory structures only up to 200 square feet and 15 feet tall, so a typical garage needs a building permit, and the City application lists “Detached Covered Parking (garage or carport)” as building coverage. The roof and any new apron both count toward impervious cover. The permit and impervious cover guide walks through it.
- Garage plans with wall locations and door sizes
- Where the new slab meets the existing driveway
- Whether a sink, floor drain or electrical runs under the slab
- Trees near the footprint, especially any 19 inches or larger in diameter
- Plans to finish or condition the space later
