Garage to ADU Conversion Cost: What You'll Actually Pay

Converting an attached or detached garage into a living unit is the lowest-cost path to an ADU, typically $120–$220 per square foot versus $220–$360 for a detached new build. For a 400 sq ft conversion that's roughly $48,000–$88,000 before regional adjustments — and often less than half the cost of building from scratch.

Why conversions are cheaper

You're reusing the three most expensive parts of any structure: the foundation, the framed walls and the roof. The budget goes into making the space habitable rather than building a shell.

• Insulation and drywall for walls and ceiling
• A kitchenette or full kitchen and a bathroom (the costliest rooms)
• Electrical upgrades and a dedicated panel or subpanel
• Plumbing runs to the main line
• Egress windows, a proper entry door and HVAC (often a mini-split)
• Replacing the garage door with a framed, insulated wall

What pushes the price up

Three things turn a cheap conversion into an expensive one: a slab that needs to be raised or re-poured for plumbing and moisture, electrical service that has to be upsized, and bringing an old structure up to current energy code. If your garage already has a sound slab and nearby utilities, you're on the low end of the range.

A bonus in California

Under California's statewide ADU law, a garage conversion requires no additional setback, and cities cannot require you to replace the parking you're removing. That removes two of the biggest obstacles homeowners hit elsewhere.