The guest bath is a jack-and-jill one, with a small room with a sink coming off the hall, then a narrow room with a toilet and tub, and then a final ... not room, but niche in the guest bedroom with a sink in it, sort of like a hotel. So not much place to put another litterbox, except for the tub itself, and as our water pressure and hot water capacity is AWESOME enough that we can take showers at the same time, we'd rather not do that!

There's a closet near the laundry room -- and we ahve a plethora of closets plus a large attic so no lack of storage space -- that might work to hold a litterbox with a cathole in the door.