As long as local zoning permits a bedroom over a garage, there's no problem at all as long as the space under the floor, including the rim joists, is insulated with closed cell spray foam such as BASF Walltite. No other method even comes close.

"Build it tight; ventilate it right!" and you should be ok. That includes good pre-planning for your heating duct locations. Be sure they are INSIDE the conditioned space, are insulated themselves, and that there are enough of them for the square footage and number of outside walls. For ventilation, that means an HRV or ERV--I forget which we used since we're right on the climate border between the two. Also, be sure to insulate the garage well, including insulated garage doors.

Our young grandson is sleeping in the bonus room over the garage. We haven't had any problems keeping the temperature normal during this long hot summer, so I assume we'll also be ok with the heating this winter. We used foam and NuWool to insulate the bonus room floor, ceiling, and walls, which are more expensive but definitely worth it to us.

Anne

I'll soon be building a garage apartment and such plans are sold all the time. I've never seen any that have climate restrictions.

My ducts will be running below the floors, through the joists, (above the un-ac'd garage) and will be insulated as well as I can figure out how.

I've heard mention of "energy rafters" which are in some way designed to allow for more insulation. They might help.

Usually the bedroom cannot be entered from the garage and any ducts that penetrate the wall between the dwelling and the garage or are exposed inside the garage must be a min. of 26 ga. sheet steel and can't open into the garage. It is best in terms of energy conservation to run the ducts in a conditioned space.

Using foam as Worthy recommended provides a good air barrier in this condition.

You might also consider resale. Some people shy away from bedrooms over the garage for a couple of reasons, the main one being noise. Couples or families with someone who has to leave early of get home very late dislike these arangements, as the vehicle and garage door noise can wake up someone sleeping above.

It's something to keep in mind.

But if you spray in foam, the noise will be greatly lessened.

A belt-drive overhead door motor is also a bit quieter than a chain drive.

Ours is belt driven, and you can still hear it even with the foam and blown in cellulose extra insulation. (I slept up there last night since I have a cold/cough.)

A kid's bedroom or office should be fine; I don't think I'd want the master BR there.

Anne

Our master bedroom suite is over our garage. The garage is slightly heated (kept at about 45-50) and is fully insulated (doors are around R20, they were the best we could get at the time). There are no issues with heating or cooling the room (it's a separate zone as well), but the floors are marble and wood and the floors (especially the marble) are cold during the worst of winter. I don't think it would be an issue if you had wall to wall carpeting. If I had it to do over I would have put radiant under the marble. We are going to have a rug bound for the bedroom and that will take care of the cold floor issue.

As far as noise goes, the garage door rarely wakes me up. I know we went for whatever type of opener was quiter, plus it's well insulated.

I'm in Maine, by the way.

As far as fumes go, fumes from what? You shouldn't leave a car running in the garage. I just start it up and backout. The smoke and CO detector has never gone off. We keep gassed up lawn mowers in the garage without issue as well.

Older houses that are nowhere near as tight as today's houses have generated a lot of complaints in the past, especially when the male half likes to fool with cars. A pot of carb cleaner really stinks, and the smell can seep deep into the house.

I used to hear a lot of those kind of complaints in the old days (usually when I was helping some buddy work on something and his wife came into the garage and pitched a fit). It usually meant retiring to some other guy's garage and checking out his beer supply. :-)