When I tour a home, I often hear, "Oh, we wanted a bathroom [over there, on this floor, off this bedroom, etc] but the area was just too small." Never underestimate the power of a good floor plan. It may be to your advantage to hire a designer who knows how much space a person needs for legroom, door swing, etc, and can coordinate a fully functional bath to fit your space. Oftentimes, if you can get plumbing and wiring to it, you can fit a bath.
If you need further convincing, check out this beautiful but tiny under-the-stair bathroom featured on Home-Designing.