This is a very common problem. Floors over garages are almost never adequately insulated to begin with. Adding two inches of rigid foam to the garage ceiling, covered with a new layer of drywall, goes a long way to solving the cold foot problem.

If you’re planning to replace the floor covering in the bedroom, then put another layer of rigid foam down first, with a new subfloor and finished floor on top.

If you’ll be redoing the finished floor anyway, consider installing electric radiant heating grids in the floor, as part of the renovations. Yes, electricity is the most expensive heating option in the long run, but it’s very convenient in a renovation situation like yours. It eliminates the need to add ducting, and directs the heat where it’s needed most. And as a small supplemental heat source, a little electric warmth won’t make a big dent in your home heating bill.