Yes, although there are exceptions. If you would like your bathroom to have a door that closes, then you don't need to keep the rest of the room open. In fact, most houses built before 1995 have one floor plan where the entire upstairs area is open except for a bedroom or two. They call this a "wing" house. A true split level house has two floors with separate entrances. The upper floor is accessible by a set of stairs or elevator, while the lower floor has its own set of stairs or elevator. A basement apartment could also be considered part of the bathroom if it has its own entrance from outside the home.