Obtain a building permit from your city's zoning department. Not all cities allow you to turn your garage into an apartment, but if yours does, you need a building permit for the job.
Estimate the budget for the conversion. Depending on the quality of building materials, converting a garage into an apartment usually costs between $25,000 and $40,000, at the time of publication.
Insulate the garage's walls, ceiling and floor. Insulation helps regulate the temperature in the garage when it gets hot or cold outside.
Install a toilet and shower if you want the apartment to have bathroom facilities. Have a professional plumber connect the toilet and shower to your home's plumbing system. Build walls around the bathroom facilities to separate them from the rest of the apartment.
Connect the garage to your existing air and heating system, and install vents that direct heat and air conditioning into the garage. If you want to avoid these costs, use a space heater or fan instead.
Contract a professional electrician to install electrical outlets and lighting. Most garages already have an overhead light, but it is usually too dim for use in an apartment.
Cover the floor with sealant or a vinyl mat. If your budget permits, install wall-to-wall carpeting to make the apartment more comfortable.
Paint the walls and ceiling. If you plan to rent out the apartment, choose a neutral color, such as white or beige.
Add appliances, such as a refrigerator and stove. If you want to let the apartment tenant use your appliances, this step is not necessary.
Put furniture in the apartment. Include a bed, dining table and chairs. Decorate the apartment with framed pictures or other items to make it feel more like a home.