Finishing the interior of a detached garage can create room for an in-law apartment or a play room your kids. You can design the space for a home office or an artist's work space. The fact that the garage is detached will create greater privacy for activities or privacy for overnight guests. In some areas, you might be able to create a rental unit as an apartment or office for extra income.
Inspect the outer walls and floor of the garage to check for cracks. All cracks must be opened and filled or repaired to protect the interior remodelling from moisture. Make plans to redo the concrete floor if needed. Consider removing the main garage door and tracking to provide ample room to work. Once you've decided to move ahead, get a building permit and ask about restrictions on creating living space in your garage. Check to see if you can turn it into a rental if you should ever need to.
Pull off all drywall or panelling attached to garage walls if interior walls were once finished. Examine the attic area for rain leakage and fix the roof and guttering before you install any wood framework or other materials. Enlist an electrician to examine how you will run electrical sockets and install light fixtures. Call a plumber to help you plan any bath drain lines or kitchen plumbing needs before you start to put up wall framing.
Install wood framing studs to all exterior walls and build divider interior walls to separate a bathroom or small bedroom in the garage area. While the framing is open, staple roll fibreglass insulation in the exterior areas to wall studs. Have an electrician run all electric wiring to outlets and housing for light fixtures. Bring a 220-volt line into the area for both an electric heater and a clothes dryer if you create a laundry area. Install drywall over the wood framing and in the ceiling. You will need to insulate the ceiling before the attic area is closed up.
Use joint compound to finish the drywall. Paint the interior walls and ceilings. Install cabinets, plumbing fixtures in the bathroom and kitchen counters, sinks and cabinets. Connect all electricity and plumbing. Buy an electric heater for the area or install stand-alone electric heaters. If you can afford it, install a heat and air system with ductwork. Put in a window air conditioner if you have no central air.
Install wood and tile flooring last. Finish with baseboard materials, hang closet and interior doors and trim the ceiling areas with crown moulding. Build wall units for books and media players or install book shelves in the living area.
Install a couple of skylights to add light and space to the garage area. Use glass bricks to build walls as room dividers inside the area to keep it as light as possible. Paint the walls white or beige to make the space seem bigger, but trim the room in paint that will match furnishings for a designer look.