Yes, you can keep the rabbit cage outside all year. However, ideally, the pet’s resting area should be in the shed or unused garage with natural light. Plus, the area must have proper ventilation. Likewise, for winter’s it must have protection with tarpaulin. However, keeping bunnies in an entirely isolated area is not a good idea. They are social animals and thus need your attention. So better is to keep them in part of the house with high traffic.