You need to read up in the coops section about cold weather coops. Patandchickens has a lot of great info on coops, and lives in Canada. She does not heat her coops.
They will need a draft free roost area, plenty of ventilation to get the moisture out, and a wide enough roost that they can sit on their feet to keep them warm. On really cold nights, a bit of Vaseline on combs and wattles will help prevent frostbite. They are in a lot more danger of this from moisture than from cold temps.
Many chickens won't go out in heavy snow, so they need plenty of coop space, preferably well over 4 sq ft per chicken. Your garage might be a very good idea, depending on your setup.
Do some searches or browse with Patandchickens as the author.