Garage conversion permits
Most cities require the homeowner to secure a building permit for this project, then prove the room has been built according to code. On average, this costs between $1,000 and $1,500. Note that some cities, like San Mateo, California, do not allow garage conversions; building one illegally will drag down the value of the home.
Interior garage changes
Turning a garage into a room requires adding a ceiling with insulation and building interior walls. This generally costs between $1,700 and $2,000, plus insulation at $1-$2 per square foot. Windows, which are required by law, cost about $1,000 each. Because the space must be comfortable, the HVAC system must be extended to the garage; installing ducts and vents ranges from $500 to $2,000.
Most garages are already wired, but a converted garage needs outlets in useful places; adding sufficient wiring and outlets for a room costs between $75 and $500.