Rabbits regulate their body temperature by growing a thinner or thicker coat so this makes it difficult to move them frequently between indoors and outdoors...
Rabbits do not cope well with temperature extremes; in the wild their burrows stay at almost the same temperature year round. They regulate their body temperature by growing a thinner or thicker coat according to the season so this makes it difficult to move them frequently between indoors and outdoors as the rabbit will either overheat or be too cold. Rabbits cannot sweat; they cool themselves by dilating the blood vessels in their ears. You can check whether a rabbit is too hot or too cold by feeling the temperature of its ears - they should feel warm to the touch. A rabbit that is too hot will probably be panting or flopping down frequently while a rabbit that is too cold may be sitting hunched up with its hair fluffed out.
If you have an indoor rabbit, it is fine to let it out in the garden for exercise during the day. If the weather is very cold, only let it out for short periods so that it is always running around and keeping warm i.e. don't let it become chilled through. An indoor rabbit is unlikely to grow a much thicker coat for the winter as it is always warm inside.
If you have an outdoor rabbit, you can bring it inside to play but only do this for short periods of time during the day. Otherwise the shock when you put it back outside may be very harmful to it. During the winter do not bring your rabbit inside at all - the sudden temperature change when you put it back outside can be fatal to your rabbit.
Indoors to outdoors
If you have an indoors rabbit and want to move it outdoors permanently, you can do this anytime after early spring when the frosts are over. Do not move a rabbit during the winter months. For the first few days ensure the rabbit is warm enough by giving it plenty of hay to snuggle in and cover the hutch with a blanket or hutch snuggle. Check on your rabbit frequently to ensure it is not too cold.
When winter comes protect the hutch permanently with a blanket or hutch snuggle or move it into a shed or outbuilding for extra protection.
Outdoors to indoors
Again, it is best to move your rabbit during the spring or summer months when your central heating is turned off. If you are using a cage, position it in a cool, shady area away from direct sunlight and radiators. House rabbits (no cage) will find their own cool spot to lie in if they get too hot.
If moving your rabbit during the winter, put it in a room with the heating turned off for the first week or so and check it frequently for signs of overheating. Always make sure it has plenty of water to drink.