1 A do-it-yourself garage remodel should cost no more than $10,000 and costs might be as low as $2,500 to $5,000. 2  Contact your municipality for information on zoning laws and necessary building permits. Turning a garage into an in-law apartment could cost as much as $40,000 to $70,000. 2  A do-it-yourself garage remodel should cost no more than $10,000 and costs might be as low as $2,500 to $5,000.