Wow! Lucky bunny and Son & Daughter-in-law!

I could link in some pages that have tons of inspiring looking homes but it could get overwhelming!

I think this page is a good starting point though, in regards to size and building materials. http://www.therabbithouse.com/indoor/designing-rabbit-cage.asp

Making a habitat that is easy to access to clean is a big plus too!

How is the rabbit currently being housed? Is he showing a preference for particular things? 

Where is their home is the new habitat going to be situated?  Is he a bunny that chews wood and that noise likely to drive them crazy?  

Do they think they might get a second rabbit eventually to live with him? 

 
Most rabbit like an area they can hide in if frightened. But they also don’t want to feel trapped in it either. This is why with things like cardboard hidey-houses for play, it often recommended to make to access points.
Rabbits also seem to like to go up on high perches to check things out. So some levels and shelves are good to have. Some take a while learning how to use them, where as other will be up there like a shot.

My friends husband built their indoor rabbit a “fairy castle”.  She is such a good bunny and very neat. They actually never close it up, it’s more a place for her to hang out, and houses her hay and litter box. It’s pretty sweet.  Years earlier, he’d built a larger castle (all wood) that housed the rabbits on one side and chickens on the other. One pair of rabbits still live part outdoors (they prefer it) part indoor, but the single rabbit lives indoors free roam. She has cat companions too and they love the her habitat also.