Vet at FirstVet answers:
Generally it is recommended to keep a rabbit indoors during the winter – rabbits are much happier indoors with company from their human family rather than sitting outside in a cage on their own.
As long as the temperature is around 10-15°C you can bring your rabbit indoors without a problem. If, however, the temperature goes below that you need to gradually introduce the rabbit to a warmer temperature, for example, by keeping it in the garage for a while before letting it into normal indoor temperature.
United Kingdom