Building a guest room in a garage requires that you weatherproof the space so it is comfortable. There are many ways cold air can leak into a garage or hot air can leak out: through gaps around windows, through old window panes, between a door and the threshold, through electrical outlets, switches and other fixtures. The garage door, too, is a major source of air seepage.
Determine where air is getting in and set about stopping it. New, double-pane windows are costly but effective. Weatherstrip or seal around windows, doors and anywhere else you find a gap such as around pipes. Batt or loose-fill insulation is a must for walls and ceilings. An exterior door set can stop air from coming in above and to the sides of doors, while a new threshold can plug gaps beneath the door.