Detached Garage: What to Inspect Before You Buy

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A detached garage often gets a quick glance during a showing: buyers open the door, check whether two cars fit and move on to the kitchen. That is a mistake, because a detached garage is a small building with its own foundation, roof, framing, electrical service and drainage, and it ages on its own schedule. Older Front Range homes in Denver, Lakewood, Fort Collins and Colorado Springs commonly have detached garages built decades apart from the house, sometimes by owners rather than contractors. Many sit at the back of the lot near an alley, where drainage, snow and soil movement hit them harder than the main house. This guide walks through what to inspect before buying, which problems are cosmetic and which are expensive, and how a standard home inspection treats the structure. Repair costs below are general 2026 estimates and vary by contractor and location.

Part of our Structure & Exterior guide — start there for the full picture →

What to Inspect on a Detached Garage

A thorough look at a detached garage covers seven areas: the slab and foundation, the roof, the wall framing and sheathing, the electrical feed and wiring, the vehicle door and opener, the drainage around the building, and the permit history. Each can fail independently. A garage with a new roof can still have a heaved slab, and a garage with a perfect slab can still have a dangerous wiring run buried a few inches under the lawn.

Under the American Society of Home Inspectors Standard of Practice, the standard applies to buildings with four or fewer dwelling units and their attached and detached garages and carports. ASHI’s exclusions list detached structures other than garages and carports, so a detached garage generally falls inside a standard inspection while a backyard shed or studio may not. State licensing rules and individual inspection agreements can vary, so buyers should confirm in writing that the garage is included, especially if it has its own subpanel or living space above.

Slab Cracks, Heave and Foundation Problems

Most detached garages sit on a simple concrete slab, sometimes with a thickened edge or a shallow perimeter footing. That simpler foundation makes them more vulnerable to soil movement than a house with a deeper foundation. Along the Front Range, expansive clay soils swell when wet and shrink when dry, and garages are often the first structures to show it.

Signs worth noting:

• Hairline shrinkage cracks. Thin, random cracks are common in almost every garage slab and are usually cosmetic.
• Displaced cracks. Cracks where one side sits higher than the other, especially more than about 1/4 inch, suggest soil movement or settlement.
• Heave. A slab that has domed upward in the middle, cracked in a spiderweb pattern or pushed the bottom of the walls outward points to expansive soil or water collecting underneath.
• Sill plate separation. Gaps between the bottom wall plate and the slab, or a bottom plate that has rotted where it sits on wet concrete.
• Racking. A garage door that binds, walls that lean or a door opening that is no longer square.

Our guide to foundation cracks explains how to tell normal shrinkage from structural movement. For a garage, patching cosmetic cracks may cost a few hundred dollars, while mudjacking or polyurethane lifting a settled section often runs $1,000 to $4,000. Tearing out and replacing a two-car slab can cost roughly $8,000 to $20,000 or more, depending on access, demolition and whether the walls must be supported during the work.

Roof, Snow Load and Leaks

Detached garage roofs are often built with less margin than the house. Some older garages use lightly framed rafters, widely spaced members or homemade trusses, and many were never designed with today’s snow load expectations in mind. A long-span two-car garage roof carrying wet spring snow can be one of the most heavily stressed structures on a property.

FEMA’s Snow Load Safety Guide notes that 1 foot of fresh snow can weigh from about 3 pounds per square foot when light and dry to about 21 pounds per square foot when wet and heavy, and it lists warning signs of an overstressed roof such as sagging roof members, bowing truss chords, popping or creaking noises and doors that no longer open or close. In a garage, a sagging ridge line, rafters pushing walls outward or cracked collar ties deserve attention. Our roof snow load guide covers how loads are calculated and what Colorado builders typically design for.

Look inside the garage at the underside of the roof deck. Dark stains, delaminated plywood or OSB, rusty nail tips and daylight around vents all suggest leaks or poor ventilation. Many detached garages have no attic ventilation at all, which can trap moisture and accelerate sheathing damage. Our guide to finding a roof leak covers common sources such as flashing at walls, vents and valleys.

Typical costs: minor shingle and flashing repairs often run a few hundred to about $1,500. Reroofing a two-car garage with asphalt shingles commonly costs $4,000 to $10,000, more if sheathing must be replaced. Structural reinforcement of undersized rafters, such as adding members or collar ties under an engineer’s direction, can add several thousand dollars.

Framing, Siding and Rot

Garage walls take abuse from car bumpers, snow piled against the siding, sprinkler overspray and soil that has crept up the sides over the years. The bottom 12 inches of the walls are where most damage hides.

When looking at the framing, check for:

• Soft or crumbling bottom plates, especially where soil, mulch or snow sits against the siding.
• Missing or damaged sheathing behind siding, visible from inside in unfinished garages.
• Notched or cut studs from past remodels, shelving or wiring runs.
• Header sag over the vehicle door, which can show up as an uneven gap at the top of the door.
• Insect damage such as tunnels or frass, particularly in older wood in contact with soil.
• Siding gaps and peeling paint that let water reach the framing.

Replacing a section of rotted bottom plate and a few stud ends is often a $1,000 to $4,000 job. Widespread rot, a failing header or walls that have racked can push repairs well past $10,000, at which point some owners consider demolishing and rebuilding. Garages are also prime spots for mold on the underside of the roof deck or behind stored cardboard, usually tied to leaks or condensation rather than anything unique to the building.

Wiring From the House: Underground Feeders, GFCI and Subpanels

Electrical work in detached garages is one of the most common sources of safety findings, because the wiring has to travel from the house to a separate building. Done well, that means a properly sized feeder in conduit or rated direct-burial cable at an appropriate depth. Done poorly, it can mean indoor cable stapled along a fence, an extension cord buried in the lawn or an overhead wire sagging into tree branches.

What to Look For

• How power arrives. Look for a conduit or cable entering the garage below grade or a properly supported overhead service. Exposed cable running across the yard, along a fence or under a walkway without protection is a red flag.
• GFCI protection. Garage receptacles are generally required to have ground-fault protection under current codes. The ASHI Standard of Practice lists ground fault circuit interrupters among the items an inspector inspects, and testing GFCI outlets is a normal part of a home inspection.
• Subpanels. A garage subpanel fed from the house is generally expected to have separate grounding and neutral conductors, with neutrals and grounds kept isolated at the subpanel and a grounding electrode at the building where required. A subpanel with neutrals and grounds bonded together is a common finding that an electrician should evaluate.
• Overloading. Shop tools, heaters, freezers and car chargers can exceed what a single 15- or 20-amp circuit to the garage was meant to carry.

Buyers planning a workshop with a welder or compressor, or an electric vehicle charger, should check capacity early. Our guide to installing a 220 outlet explains what higher-voltage circuits require, and our EV charger installation guide covers load calculations and typical costs. If the house panel is already full, an electrical panel upgrade may be needed before a garage feeder can be enlarged.

Before any trenching for a new or replacement feeder, buried utility lines need to be located; our guide to calling 811 before you dig explains how the request works in Colorado. Running a new underground feeder to a detached garage often costs $1,500 to $5,000 depending on distance, trenching conditions and wire size, and adding a subpanel commonly adds $1,000 to $3,000.

Garage Doors, Openers and Springs

The ASHI Standard of Practice lists garage vehicle doors and garage vehicle door operators among the items inspected. A detached garage door can be the most heavily used moving part on the property, and it can also be one of the most dangerous if the springs or safety features fail.

During a walkthrough, check that:

• The door opens and closes smoothly without jerking, scraping or stalling.
• The automatic reverse works when the door meets an obstruction, and the photo-eye sensors near the floor stop and reverse the door when the beam is broken.
• Springs are intact, with no visible gaps in torsion springs or stretched extension springs. Extension springs should have safety cables running through them.
• The bottom seal is intact and the door does not leave large gaps at the floor.

Opener problems are often modest repairs; our garage door opener repair guide covers common causes and costs. Spring work is different. Springs store a large amount of energy, and our guide to garage door spring repair explains why it is generally a job for a trained technician. Replacing an entire door and opener on a two-car detached garage commonly runs $2,000 to $5,000 or more for an insulated model.

Insulation, Heat and Year-Round Use

Many buyers picture a detached garage as a future workshop, home gym or hobby space. Most were built as unheated shells, and that matters both for comfort and for how the building ages. An uninsulated garage on the Front Range swings from well below freezing on a January night to hot in a July afternoon, and stored paint, electronics and water lines suffer accordingly.

Points to check when the garage is going to be used for more than parking:

• Wall and ceiling insulation. Open stud bays and an uninsulated ceiling are normal; insulation can be added, but it also hides the framing from future inspection, so deal with rot and wiring first.
• The vehicle door. A thin, uninsulated steel door is the largest heat-loss surface in the building. Our insulated garage door guide compares door constructions, and our guide to the garage door seal covers bottom, side and top weatherstripping that keeps out snow and rodents.
• Heaters. Fuel-burning unit heaters need proper venting, clearance to combustibles and a gas line installed with a permit. Electric heaters need a dedicated circuit, which brings the feeder capacity question back into play.
• Water lines. A hose bib or utility sink in an unheated garage needs a way to drain or a frost-free design so the line does not freeze and split.

None of these are deal-breakers on their own, but together they can turn a “free” workshop into a project worth budgeting for: insulating and drywalling a two-car garage plus a heater and a dedicated circuit can easily reach $8,000 to $15,000 or more.

Drainage, Fire Safety and Carbon Monoxide

Detached garages are often built at the low end of a lot, next to an alley or at the bottom of a driveway that slopes toward them. Look for soil and pavement that slope toward the building, downspouts that discharge against the walls (if the garage has gutters at all), water stains on the slab inside and erosion along the foundation edge. Many garage problems, including slab heave, bottom-plate rot and roof sheathing damage, trace back to water. Regrading, adding gutters and extending downspouts are among the least expensive fixes, often a few hundred to a couple thousand dollars.

The U.S. Fire Administration notes that flammable liquids, combustible storage, gas appliances and electrical tools all contribute to garage fire risk, and recommends a heat alarm rather than a smoke alarm in a garage. Its guidance also suggests storing gasoline, paints, propane and varnishes in a shed away from the home. A detached garage creates distance from the house, but it does not eliminate the hazard, and a garage close to a house or neighboring structure can still threaten them.

Carbon monoxide is less of a concern in a detached garage than an attached one, but not zero. The CDC advises always opening the door to a detached garage to let in fresh air when running a car or truck inside. Anyone using the garage as a workshop with fuel-burning heaters or engines should ventilate well.

Permits, Additions and Living Space

Ask the seller and check with the building department about permits for the garage itself and for any later changes: a second story, a converted studio, plumbing, a large subpanel or an enlargement. Unpermitted living space above or inside a detached garage can trigger questions from appraisers, lenders and insurers, and a city may require corrections before the space can legally be used. Zoning rules may also limit how close a garage can sit to property lines and whether it can contain a dwelling unit.

A home inspection identifies visible defects but is not a permit or zoning review. Our guide to hiring a home inspector covers how to confirm scope and when to add specialists such as an electrician or structural engineer, and the structure and exterior hub gathers guides on roofs, foundations and siding.

When a Garage Problem Changes the Deal

Most detached garage findings are maintenance items. A few are worth negotiating or getting a specialist opinion on before the inspection objection deadline:

• Displaced slab cracks combined with racked walls or a binding door.
• A sagging roof line or rafters spreading the walls apart.
• Improvised underground wiring or a subpanel with obvious grounding problems.
• Extensive rot at the bottom of the walls.
• Unpermitted living space or plumbing.

Getting a written estimate for each significant item gives buyers a concrete basis for a repair request or credit.

References

• ASHI Standard of Practice for Home Inspections — American Society of Home Inspectors
• Snow Load Safety Guide (FEMA P-957) — Federal Emergency Management Agency
• Basement and Garage Fire Safety — U.S. Fire Administration
• Carbon Monoxide Poisoning Basics — Centers for Disease Control and Prevention

Not sure whether that back-of-lot garage is a bonus or a project? Request an inspection and have the slab, roof, wiring and door checked before your objection deadline.