Secondly, it's about a 50/50 chance in our neighborhood that the garage will be used to store vehicles. The other half of the time I've seen garages used for general storage, or for a workshop of some kind (my girlfriend has a ton of wood working equipment in the garage).
Speaking from experience here.
Also, as of 2011-ish, CO detectors are required by law in CA if you have an attached garage, a fireplace, or gas heat.
...Are you aware that there's a door between a house and its attached garage, much like the one between the front of the house and the street outside?
Your description sounds like you think garage doors open directly onto the living room.
Also I never had an attached garage when I lived in Boston, Denver, and in the Colorado Rockies, so I'm not so sympathetic to this "oh, it's cold!" argument.
You could have saved a lot of time if you'd said that in the first place.