In tiny houses, you will typically find at least one interior door. It might be for a pantry, closet or back room. Most likely, that door gets installed to create privacy for bathroom users.
Why not get door-creative? Take a look at these bathroom sliding doors, built into three homes under 227 sq. ft. They are sweet alternatives to standard swing-out doors.

One Georgia cottage, built on brick piers, features a pocket door which slides into the wall to reveal a flush toilet, sink and shower. (Youtube, Ben Hartel)