A cold floor in a room over a garage is a common complaint.  Often, the walls are insulated, the attic is insulated, and the ductwork is in good condition to the room.  Here is the culprit….

This view is looking at the insulation in the floor of the room (or the garage ceiling, depending on your perspective).  In the crawl space of the house, the insulation is held tight to the floor with spring steel.  Here, over the garage, it was left to rest of top of the drywall.  Insulation only works if it is in contact with the surface that is supposed to be warm.  In this case, the insulation does a good job of insulating the garage ceiling, but a poor job of insulating the room floor.  As the floor warms up, it heats the air in the space between the insulation and the floor, which undergoes thermal convection (warm air cycling with cold) and cools off the floor.  Solution:  blow insulation above the existing insulation to fill the space.  Or, when you are building a house, be sure that the insulation fills the joist cavity.