In my experience with two new houses, both with rooms over the garage, those rooms are cold period. Add in pipes and a bathroom over the garage and this doesn't surprise me. I would say that heat tape would make a difference, but finding someway to put heat into the garage is a good step. One time, my pipes froze, and I ran a kerosine heater in the garage, and the pipes slowly became unfrozen as the heat rose to the ceiling. If I had to do it again, I wouldn't put any rooms, or plumming over the garage because they are cold in winter, warm in summer!!! Good luck!