OH, I remember I was so worried for my hens when they had their first winter. If your chickens have small combs, that's a good thing because the combs can freeze. The main thing is keeping them out of the wind and keeping them from getting wet. Also, if they can roost on a 2x4 board (broad side up), then they can sit on their feet to keep them warm. I'm in Ohio, and we get some extremely cold temps, like you do.
If you can keep a heat lamp in your coop, thats great because those really help.....and it can help keep their water from freezing too. And if you use the deep litter method, you can go through the winter without cleaning out the coop entirely....but good ventilation is important.
Using the garage is up to you......if you've got a good coop, maybe you can just use a heavy duty outdoor extension cord to put a heat lamp in it.....