happykate... our cat bathroom is the same! We have a pantry off the kitchen and the back wall has a 2x2 opening in it (6 inches off the floor, with a small step) for the cats to go thru. That entire wall is pantry shelves, so we lost hardly anything to put it there. The other side is the attached garage, where we have a door that lets us in to store cat supplies, misc. stuff at the end where the stairs meet the floor, and the 2 litter boxes and trash. We have a 5W CFL that stays on 24x7 for them and a mini-split off the A/C system in the ceiling. For vacuuming, we just pass the hose through their doorway and walk around to our door. I consider it the most brilliant extra we thought of at framing time!!!

My space was funtional. I'm not sure of the dimensions of the closet since I don't live there anymore. It was the width of a regular size hallway. The side shelves were probably 12" deep. I just went to Lowes and bought the wall shelving brackets and had the shelving boards cut to size there. You could always go shorter if you wanted. In my current house, I have a wall in my pantry that was going to be left bare because the 12" shelves were going to be too big for the space. I had the guys build 6" instead. Those shelves have turned out to be really useful for storing the mega-size spice containers I get from Sam's Club.

You might also hinge those shelves, after you've put the casters on; I think that would make it easier to roll them around. Basically, anywhere you can find info about framing out the space under the stairs, even if it's for a closet, (installing a header, etc.), could help you frame the opening w/ a door frame. Then, all you need to do is make a bookcase to the space that's left. Put a wide frame on the front of the cabinet (w/ molding) so that the molding overlaps and hides the opening. under-stairs collector's cabinet http://www.diynetwork.com/diy/shows_dwsp/episode/0,3110,DIY_28376_47620,00.html under-stairs wine rack http://www.diynetwork.com/diy/shows_ttd/episode/0,2046,DIY_14339_26671,00.html understairs closet http://www.diynetwork.com/diy/hi_storage/article/0,2037,DIY_13934_5211383,00.html how to build new storage under your stairs, from Popular Mechanics magazine, w/ illustrations and pictures http://www.popularmechanics.com/home_journal/woodworking/4212966.html (I Googled the words: under stairs closet build and got several things that look useful--I know you didn't want that sort of advice, but I was surprised at the quality of what I found ) And a note on a forum: " I was under the impression if you were able to access the bottom of the stairway the walls and ceiling should be sheetrocked with 1/2" rock?" -- "James, you are correct but I'm not sure which code specifies those requirements."

I'd use it as storage. Use full extension pull out slides and build your shelves to fit. Either leave the face of the shelves open or add a panel to accent the interior decor so that the shelves are hidden from view. Or do something like this http://pipsplate.files.wordpress.com/2006/08/dsc02214.gif or this http://unclutterer.com/wp-content/uploads/mt/070531-stairs.jpg or this http://farm1.static.flickr.com/46/114269424_7b27625bc3.jpg Jim