Hi there,
I think it’s great you are working to bring them inside. A great starting point is to check out the sections here: https://binkybunny.com/house-rabbit-information/
There is a section on litter training, habitats, bunny proofing, etc.
For litter training, it will be important to get them neutered. Once neutered, most rabbits are pretty easy to litter train. It’s not so much training as taking advantage of an instinctive behavior to use one corner of the pen as the toilet area. Un-castrated rabbits are usually hard to litter train completely. They may still use a litter box a lot of the time, but will also mark other places.
As far as keeping them happy and stimulated inside, I think you could do a lot by bringing in safe branches and twigs for them to chew on inside, as you probably already have a sense of what is safe for them to eat from the garden. I would be best if the buns could be in an area with lots of human activity. Garages don’t tend to be ideal because people don’t spend a lot of time in there, and the temperature fluctuations can be extreme. You especially need to worry about how hot it will get.
Having a large bunny-proofed roam to roam in is recommended. With suitable flooring and enrichment (tunnels, hides, some toys), most buns are very happy in this set up. You can also do things like provide a digging box if your buns like to dig. You could also still give them some run around time in the garden when you can but having their primary home indoors will definitely be safer for them.
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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.