The other problem is there is really no good place to put this bathroom! I don't want to take a chunk out of the kitchen. I don't want to take a chunk out of the dining room - those rooms are relatively square. There is the office/playroom/bedroom but I don't want to carve that up into either a giant bathroom or two smallish room with a hallway (big waste of space).
So I am eyeballing the space under the stairs - it's a huge space albeit long and narrow with a sloping ceiling, but space that is not well used. I am trying to preserve a shallow closed in the office/playroom/bedroom and cut another door into the wide part of the opening. I have visions of built ins and a place for the litter box (those cats don't need much head room!)...
The real sticking point is I need the smallest sink I can find - pedestal or wall mounted. I would love a tiny vanity that had a door for storage, but they all seem to be too wide. I found this article that is very helpful
Ask This old House: Itsy Bitsy Bathroom. Now I've got to go and measure things...and see how our space adds up.
Itsy Bitsy Bathroom
Q: What is the smallest size possible for a half-bath?
— Celia, By Email
A: Tom Silva replies: I've put half-baths, sometimes called powder rooms, in some really small spaces, including closets and under stairs. But I've never tried to figure out just how small I could go without upsetting the building inspector. Until now. And just so you know, the cost of fixtures or installation is not a factor in this exercise.
The two fixtures in a half-bath—a sink and a toilet—both need a certain amount of space just so you can use them comfortably. The International Residential Code (IRC) requires a minimum side-to-side clearance of 15 inches from the centerline of a toilet to the nearest wall. So the bathroom's narrowest dimension is 30 inches. IRC also requires at least 21 inches of clearance in front of the toilet bowl. There are bowls that stick out only 20 inches from the wall, which means you need a minimum footprint of 8 ½ square feet just for the toilet.
As for the sink, a small one mounted in a corner opposite the toilet can project just 11 inches from the wall. The IRC specifies you leave at least 21 inches of space for someone to stand in front of a sink. But since you're not likely to have more than one person in there at a time, this space can overlap with the space in front of the toilet. Total room length, therefore, could be 52 inches. If equipped with a pocket door, which doesn't require any swinging space like a hinged door, your tiny room could occupy a very space-efficient 11 square feet.
Don't forget about headroom. You'll need at least 7 feet, except above the toilet, where it can be as low as 6 feet 8 inches. All of this works in theory; just be sure to check with your building department for local code requirements before you put theory into practice.