It’s recommended to have the primary living space (where the bun spends most of it’s time) be at least 12 square feet, plus an additional 50 sq feet (minimum) for exercise space at least 4 hours a day. More space for more time is even better.
Rabbits need room to run at full speed at least some of the day to live happy and healthy lives. The idea with the main living space is that they should be able to take at least 3-4 hops without running into a wall.
Building levels up is a great idea, I have a multi-level condo for my bunnies (but I actually leave it open most of the day so they have free-roam in my apartment).
So 25 sq feet would be awesome for the primary living space, and then giving an additional larger area (such as the rest of room the enclosure is kept in) to run in for 4 hours a day (esp timed to when they are most active) would mean a happy rabbit.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.