Assess the garage's condition before converting it into a room to rent out. Check for mold and moisture problems, and get rid of any pest problems first. You should also investigate the insulation and ventilation, check the conditions of the floors, and fill in cracks or holes in the foundation.
To turn a garage into a rental space, it is best to replace the garage door with French doors or a solid wall with a built-in door. The renters will feel more secure and comfortable with this new set-up. It is also a good idea to add windows for natural light, ventilation and views of the outdoors. Most garages don't have insulation, so it is important to add it. Proper insulation in the walls allows for better moisture and pest control, as well as maintaining a comfortable temperature in the room.
If it is possible, try to set up the garage rental space for air conditioning and heat. You may also need to add more electrical outlets to be added to the walls, which you should hire a professional to do. Have an adequate number of lights and switches, considering what fixtures the renter might need. For a rental space that is a small apartment, you may need to add plumbing to allow for a sink and small kitchen space, as well as plumbing for a small bathroom.