Bunnies actually don’t mind the cold much. When I was a teen, it was believed that indoor air was bad for bunny lungs* so they were always kept outdoors winter was fine for the bunnies. They just huddled down inside the wood part of their hutches. Bunny fur is extremely warm.
It was summer weather when we had to worry. We went batty keeping the poor furry things cooled down.
Now I’m not saying a garage is a good place for a bunny. First off, I would be very concerned about air quality. Garages sometimes store nasty chemicals. Plus if it is used as a garage, there are nasty chemicals. The bunny is likely to get lonely in the garage, who wants to go play with the bunny in the cold garage? When I was a teen. going out to be with my bunny on our porch was bad enough in winter, but a depressing garage would have been awful.
The garage just seems like all the down side of outdoors, with none of the benefits of outdoors.
I would choose a sturdy outdoor hutch over a garage. That way at least your bunny gets to enjoy seeing nature and fresh air.
*I wonder if our indoor air quality used to be much worse or if this was always a total myth. My mom tells me that when she was a girl my grandmother killed several canaries in their house by cooking with pans that gave off toxic fumes when overheated. After a while they just gave up buying canaries.