For an attached garage, there has to be a fire barrier between the garage and the house extending from the floor all the way to the top of the roof. There's probably fire rated sheetrock on the wall between the garage and the house, and then they typically run the sheetrock on up all the way to the peak of the roof, nailing it against the rafters or trusses. If that's what you have, you're probably ok, but if your garage ceiling is already sheetrocked I suppose it's possible that layer of sheetrock is the fire barrier, so you need to establish where that barrier is if it exists.
Secondly, is the area above the garage designed for storage? I assume it's probably roof trusses up there, and regular old trusses usually aren't designed to hold any amount of weight (beyond maybe a few boxes of Christmas ornaments or stuff like that.) If you're thinking of storing car parts or anything heavy up there, you should probably talk to a local contractor or possibly even a structural engineer to see if they can take the load.
