Attic space over an existing attached garage can provide building room for a bedroom or game room. In a two-storey house, you can usually access this space from an upstairs room or hallway. You will have to remove plasterboard to enter this space. Or, you can enter this space by cutting a hole in a garage ceiling if it's over a garage attached to a single-storey house. Make sure you have ample space before you invest in building there. Adults should be able to walk around in a 2.7 x 3.6 m (9 x 12 feet) area in the attic space with sufficient headroom.
Design the floor plan and electrical wiring needs of the new attic room. Cut a temporary door into a room or hallway adjacent to the garage attic. Alternatively, build a stairway up from a garage below. In most cases, there will be no flooring in place over the garage area. Run wiring between floor joists before you install sub-flooring. Install all wiring for overhead lights, wall plugs and outlets. Because garage space is unheated, insulate well before installing particle board or plywood sub-flooring.
Frame walls and ceiling to accommodate plasterboard. Insulate these areas with fibreglass roll-type insulation and staple into place. If the room has slanted ceilings on the sides, there should be usable attic space behind side walls once those side walls are squared off. Builders refer to this attic space behind a finished wall as knee space. Install a permanent doorway into a hallway or other room. Nail plasterboard into place and finish the room appropriately for final painting.
Build handmade bookcases or storage areas last. Use space in low-ceiling areas for built-in storage boxes with lids made from wood. Make upholstered cushions to top storage boxes to create seating if head space permits.
Install wooden flooring or lay carpet over sub-flooring. Install light fixtures, switches and electrical wall outlets. Add skirting, trim and moulding to the room.
Bring oversized furniture into the room before installing a finished doorway. An air hockey table or billiard table might require extra door width coming into the new room. Consider placing the door's overhead support beam, door jamb kit or threshold at the room's entryway after such pieces are brought in. You might consider adding extra insulation or a sound barrier to a wall adjacent to other living quarters if the room's activities will be noisy.
Be careful about running a car in the garage beneath due to carbon monoxide. On cold days, back the car out of the garage to let it warm up. Otherwise, fumes may rise into living areas above the garage. Make sure the garage area below is vented properly and no flammable materials are stored below your new room in the garage. Put flammable materials in an outside storage building away from the house.