Rabbits can actually handle the cold a lot better than the heat. My rabbits have been outside in the winters but they always are protected from wind/drafts and wet. Rabbits are fine in the cold if it's nice and dry and they have somewhere to cozy up, like a hidey house or another bunny.
This year we have a shed for the bunnies and a heating system, but they don't need it to survive.
Just make sure the bunny doesn't have frozen water in the water bottle, and that the spout, if it's metal, is dry. Tongues can freeze to wet metal if it is cold enough, which has disasterous consequences. Also make sure your bunny has melted water regularly because ice isn't that great for drinking.
