We turned the unused space under the stairs into a room for the cats’ litter.
We built a ramp out of plywood and covered it with indoor-outdoor carpet. It runs up from the pet door we installed through the wall to a level area for the litter tray. Bi-fold doors on the end of the little “room” keep the cats out of the furnace room and insulation around it keeps the smell from the litter in the room.
By installing a little Radio Shack fan inside a 2” PVC pipe with plumbing-type elbows, vented into the ceiling of the furnace room, into the furnace exhaust’s plenum, provides enough draft to duct excess odor all the way up to the attic and, eventually, through the roof, helping keep down the smell.
The tray is at waist height, so is easier to clean out, and the space under the ramp is handy for a trash can for cat waste, a can of extra litter, their cat carriers, and a few old litter trays.