When placing fixtures in your bathroom layout, think about how they're used and in what order. The sink, for example, should be positioned closest to the door, because it's the last stop in most bathroom routines. Having to squeeze by other fixtures on your way in and out the door can be inconvenient. The toilet, tub, and/or shower can be farther from the door and enclosed in separate compartments for greater privacy, if you wish.
How many "wet walls," or walls that contain plumbing pipes, will there be in your bathroom? The fewer wet walls you have, the less costly your plumbing bill will be.
One-wall designs are the simplest and require the fewest plumbing fittings. That makes them the most inexpensive. Keep in mind, however, that design possibilities are limited with a one-wall layout, and floor space may not be used efficiently.
A design with plumbing in two walls offers more floor area and storage space around the sink. In a smaller bathroom, especially, it may be worth the extra cost.
These designs offer the greatest flexibility, but they require more complex and expensive plumbing, as well as more floor space.