Interestingly, all that's required for a space to be listed as a bathroom
here is that it have a toilet in it. I recently saw the smallest "bathroom"
in the world! Remember the toilets in kindergarten? Smaller than that.

And now there's a new japanese toilet with a built-in sink on the
cistern, so you only need the one unit. You can stuff the whole
thing into a 28" x 40" closet..