Planning a Detached Garage: What to Decide Before the Concrete Is Poured

Most of the decisions that make or break a detached garage happen before a single wall goes up. Once the slab cures, its size, location, thickness, slope and everything buried inside it are set for the next 40 years or more. So before the concrete truck backs in, you need to settle five things: where the garage sits on your lot, how big it will be, which foundation type your climate calls for, what utilities run under the floor, and where water will go when it rains.

That sounds like a lot, but it comes down to a handful of choices. Get them right on paper and the build goes smoothly. Get them wrong and you're paying someone to saw-cut fresh concrete, or parking a pickup that sticks out past the door. Here's how to work through each one.

Start with zoning, setbacks and the building permit

Nearly every town requires a building permit for a detached garage, and local zoning decides where it can go. Check with your building department first, because setbacks and size limits shape every other decision on this list.

Most US codes only exempt accessory buildings of 200 square feet or less, and even a one-car garage is bigger than that. Expect rules on side and rear setbacks (often somewhere in the 3 - 10 foot range), distance from the house, maximum height, lot coverage and total footprint. Some towns also cap the garage size relative to the house or require matching siding and roof pitch. If you have an HOA, its rules apply on top of the city's.

The permit set usually needs a site plan showing property lines, easements and the garage location, plus a foundation detail. Many homeowners hand that part to a contractor who pours concrete foundations for garages regularly, since they already know what the local inspector wants to see on the drawing. And before anyone digs, call 811 so buried gas, power and cable lines get marked.

How big should a detached garage be?

A two-car detached garage should be at least 24 x 24 feet if you want room to open doors and walk around both vehicles. A 20 x 20 layout is the bare minimum and feels tight fast. When in doubt, go one size up, because you can add shelves later but you can't add slab.

Measure what you actually drive. A crew cab pickup with a long bed can run close to 20 feet, so a 24-foot depth is the real minimum for trucks, and 26 - 28 feet if you want a workbench at the back wall. Door height matters too. A standard 7-foot overhead door won't clear many lifted trucks, vans or roof racks, so 8 feet is a safer pick. If a car lift is even a maybe, plan a ceiling of 11 - 12 feet now, because the slab and walls have to be designed for it.

Choosing the right detached garage foundation

In mild climates, a monolithic slab with thickened edges is usually the cheapest and fastest option. Where the ground freezes, you'll typically need footings below the frost line with a frost wall, with the floor slab poured inside it.

A monolithic slab is poured all at once, with the edges dug deeper (often 12 inches wide and 12 - 18 inches deep) to act as the footing. It's quick and needs only one inspection and one pour.

A frost wall foundation puts footings below your local frost depth, which ranges from a few inches in the South to 4 - 5 feet in the northern states. A short concrete or block wall rises from the footing, and the slab sits inside. It costs more, but it keeps wood framing higher off the ground and handles sloped lots better. Some northern towns allow a floating slab for smaller detached garages, and a frost-protected shallow foundation with rigid insulation is another option. Your building department will tell you which ones it accepts.

How thick should a garage slab be?

Four inches of concrete is the standard garage slab thickness for cars and light trucks. Go with 5 - 6 inches if you'll park heavy pickups, an RV or a camper, or install a car lift.

The code minimum is 3.5 inches, but almost nobody pours that thin for a garage. In freeze-thaw regions, garage floors call for air-entrained concrete of at least 3,500 psi, and many contractors order 4,000 psi to handle road salt dripping off cars. For a two-post lift, check the manufacturer's specs before the pour. Most want a minimum thickness and strength right under the posts, and it's easy to thicken that area now and very hard later.

Reinforcement is another early call. Rebar in a grid or welded wire mesh helps keep cracks tight, but only if it sits in the middle of the slab on chairs, not flat on the gravel. Control joints matter just as much. Plan them every 8 - 12 feet on a 4-inch slab, so a 24 x 24 garage is usually cut into four squares.

What goes under a garage slab?

A garage slab should sit on 4 - 6 inches of compacted gravel over firm, compacted soil. Add a vapor barrier if you'll ever heat, insulate or finish the space.

Topsoil, roots and organic material have to come out completely, since they rot and settle. Soft spots get dug out and replaced, and any fill goes in thin layers that are compacted one at a time. If your lot has heavy clay or old fill, a soil test is cheap insurance. For a heated workshop, consider rigid foam under the slab too. It's the only time you'll be able to put it there.

Utilities to rough in before the pour

Anything that runs under or through the slab must be in place before the concrete goes in. That means electrical conduit, water lines, floor drains, radiant heat tubing and empty sleeves for the future. Adding them later means cutting concrete or trenching around the building.

• Electrical conduit from the house, sized for a subpanel if you might add an EV charger, welder or air compressor;

• One spare empty conduit, because it costs a few dollars now and saves a trench later;

• A water line buried below the frost line if you want a utility sink or hose bib;

• A floor drain or trench drain, but only after checking local rules, since many towns restrict where garage drains can discharge;

• PEX tubing for radiant floor heat if there's any chance you'll heat the space;

• Anchor bolts set in the wet concrete at the spacing your code requires, kept clear of door openings.

Before the pour, take photos of every pipe and conduit with a tape measure in the shot. Years from now, when you want to drill into the floor, those pictures will tell you exactly where not to.

Should a garage floor slope?

Yes. A garage floor should slope toward the overhead door or a floor drain so melting snow and wash water run out instead of pooling. A common target is about 1/8 inch per foot.

Don't overdo it, though. Too much slope makes cabinets, tool chests and car lifts hard to level. Many builders keep the total drop to 1 - 2 inches from the back wall to the door. The top of the slab should also sit a few inches above the finished grade outside, so water from the yard can't flow in under the walls.

Plan drainage around the garage

The ground around a detached garage should fall away from it by at least 6 inches within the first 10 feet. Gutters and downspouts should send roof water well away from the slab and from your house.

A new garage adds hundreds of square feet of roof and pavement, and that water has to go somewhere. Watch for spots where it could get trapped between the house and garage, or run straight toward a basement wall. On lots with clay soil, a high water table or a garage set into a slope, it pays to bring in residential property drainage specialist Site Prep before the forms go in. Their crews handle excavation, pad grading and French drains in one job, so the site is shaped to move water away before the foundation is built on top of it.

Driveway, apron and access

Decide how the driveway reaches the garage before the slab is poured, because the door height, apron and slab elevation all have to line up. Also make sure concrete and delivery trucks can get to the site.

The apron is the short concrete section in front of the doors. It should slope slightly away from the garage and be separated from the slab with an isolation joint. A loaded concrete truck weighs well over 60,000 pounds and can crack an existing driveway, crush a septic line or rut a lawn. If the garage sits far back on the lot, a pump truck may be the safer choice.

Pre-pour checklist

Run through this list with your contractor a few days before the pour.

1. The permit is approved and the footing or pre-pour inspection is scheduled.

2. Forms match the approved size and setbacks, checked against the property pins.

3. The gravel base is compacted and the vapor barrier is taped at the seams.

4. Rebar or mesh sits on chairs at mid-depth.

5. All conduit, pipes, drains and sleeves are in place and photographed.

6. Slope, door openings and anchor bolt locations are marked on the forms.

7. The concrete mix, psi and air entrainment are confirmed with the supplier.

8. There's a plan for control joints and curing, especially in hot or cold weather.

A garage slab is one of the few parts of a home project with no real undo button. Spend an extra week on these decisions and the rest of the build, from framing to the overhead door, goes up on a base that's exactly the right size and ready for whatever you park on it.