How much reloading are you planning on doing?
If it's not a lot - say 50 rounds a week, then the garage will be fine, particularly if you choose to do them when it's relatively good weather.
Alternatively, if you're planning on doing a lot, and spending hours at a time in an ice cold garage, the spare room might be the better option.
Personally, I wouldn't consider the spare room option, it's part of my hobby and I don't mind if I have to "rough it a bit".