Rabbits need an enclosure that is at least 4 times the length of the rabbit. Typical enclosures are x-pens, multiple-level condos, large dog cages, a room, and even a whole house.
A puppy training folding fence 26 or more inches tall serves as a simple pen, called an exercise pen or x-pen. It provides more free space and is easier to clean than some traditional cages. Multiple x-pens can be joined to enlarge the enclosed area. An x-pen’s portability is useful for travel with rabbits and for introduction and bonding.
A customizable pen is a multiple-level condo. The rabbit condo can be made by using cable ties and storage cubes available at home supply stores. Corrugated plastic and carpet remnants are commonly used as flooring. A multiple-level condo provides the rabbit plenty of jumping options and variety. The owner chooses the dimensions and layout.
Another type of pen is a large dog cage. House rabbit organizations caution against using a grid floor, as this will cause sore hocks (sores on the bottom of the rabbit’s back feet). Carpet or linoleum can be added to a cage that has a grid floor to protect the rabbit’s feet, or sometimes the grid can be removed.
A rabbit-proofed room outfitted with a litter box, toys, and food can also serve as an enclosure. Rooms shared with humans, such as a bedroom or a kitchen, are typical. Where it is practical to rabbit-proof an entire house, a house rabbit can freely roam the house as cats and dogs do. Depending on the rabbit, this could require additional litter boxes, possibly one per room or per floor.
Within its enclosure, a house rabbit is provided a small shelter to hide and rest in. Cardboard boxes work well both as shelters and chew toys so long as they do not provide sufficient elevation that the rabbit could jump out of a pen.
taken from : http://rabbit-rabbits.com/2009/05/rabbit-housing/