Code change: many cities are now blocking garage conversions
Some jurisdictions — California most notably, but increasingly others — have minimum off-street parking requirements that get violated when a garage becomes living space. Before you commit to a designer contract, call your local planning department and ask: "If I convert my attached garage to a bedroom, do I need to provide replacement off-street parking, and what counts?" The answer determines whether the project is feasible at all.
In jurisdictions that allow conversion, ADU (accessory dwelling unit) regulations sometimes provide a path that other conversions don't. A garage converted to a permitted ADU can sometimes legally rent — turning the conversion into both living-space gain and rental income. Worth checking before scoping this as just a "extra bedroom."
See also: Room Addition Cost — $130–$400 per sqft. The alternative when keeping the garage matters more than saving on construction.
Garage slabs slope to drain — and that fights you in living-space use
Garage slabs are poured with a 1–2 inch slope toward the door, so cars can drip dry without flooding. A living space with a 1-inch slope across 20 feet feels uncomfortable to walk on, and any furniture sits cock-eyed. There are three options: (1) leveling compound to bring the slab flat ($1,500–$3,500), (2) raising the floor with a sleeper system + new subfloor ($3,000–$6,000), or (3) full slab replacement ($8,000–$15,000).
Sleeper systems (option 2) are the most common because they also create space for vapor barrier, insulation underneath the floor, and electrical chases. The downside is they raise the finished floor 3–6 inches, which can make the existing house-to-converted-garage threshold awkward. Plan the threshold detail before framing.
Insulation is where comfort and resale value actually live
Garages are uninsulated by code. Converting to a heated/cooled space means insulating all four walls (one of which is the new garage-door-replacement wall, the other three are existing exterior + interior walls), the ceiling, and ideally the slab perimeter and underneath the floor. Spray foam in the rim joist, R-19+ in walls, R-38+ in ceiling, R-10 rigid foam at slab perimeter is the standard package.
Cheap garage conversions skip the floor insulation. The room then runs 10°F cooler than the rest of the house in winter, and the cost to keep it comfortable on the existing HVAC system is high enough that homeowners often stop using it. Spend the $1,500–$3,000 on floor insulation up-front.
Compare: Attic Conversion Cost — $50–$225 per sqft. The other "convert existing space" option with different structural math.
HVAC capacity is almost never enough as-is
The existing forced-air system was sized for the existing conditioned square footage. Adding 400 sqft of new conditioned space typically requires either upsizing the system (when the existing system is older) or adding a dedicated mini-split for the converted garage ($3,500–$7,500 installed). Ducted extensions from an undersized system result in the converted room never reaching set-point and the rest of the house overheating to compensate.
On garage conversions specifically, mini-splits are almost always the right call. They're relatively easy to install through the new exterior wall, give the converted room independent thermostat control, and avoid disrupting the rest of the house's duct sizing.
Resale value: garage conversions usually subtract value, except in specific cases
In most U.S. markets, a garage is worth more to buyers than the converted-to-bedroom equivalent. Appraisers downgrade homes that lost their garage. The exception: very dense urban markets where parking is irrelevant and bedroom count drives price (some Bay Area, Brooklyn, central Boston neighborhoods). In those markets, a permitted ADU conversion can add 8–15% to resale value.
In suburban markets, do the conversion only if you're solving a real space problem (need a 4th bedroom, need a home office) and plan to use the room for a long time. Don't do it as a resale play in a market where buyers expect a garage.