Our garage (car doesn't live in it) is flippin freezing in winter. It's concrete base and corrugated metal roof.
Ours have a wooden playhouse which is quite nice and warm, but I brought mine in for the night in winter.
You can insulate with carpet on top of the hutch.
Snugglepads (a microwavable flat silicone disc in a fleece cover)
Hot water bottles (again in fleece covers)
I use rubber car mats, cardboard (insulating) and newspaper -to make cleaning easy and absorb bodily fluid.
I put straw at the back of the haybox to insulate (straw and hay costs the same where I buy it. It gets thrown out daily, but straw is more voluminous).
Lots of hay to eat and keep them warm. When I bring them in for the night their ears and paws are always warm, they do flatten the hay bed. But where they lie is snuggly. (They don't cuddle together, none of that nonsense) 
I personally wouldn't keep my boys out at night. It's nice to settle them for the night when it's frosty outside, knowing they are snug in the dining room . Need to make sure they don't get too warm though.The change in temperature is bad for them.
Then I have to spend half an hour getting their house ready for the day