Guest baths, which get only occasional use, can focus on style, as in this example with its dramatic asymmetrical sink and bold, geometric tile patterns on walls and floor.

With only two fixtures to accommodate, half-baths (aka powder rooms) practically design themselves. You can fit a sink and a toilet into less than 20 square feet and still meet building codes.

Half-baths don't need much storage, natural light usually is not a priority — a window may even detract from privacy — and less counter space cuts down on clutter. But there are some practical considerations, starting with the entrance. If possible, the door should swing into the room rather than out. (A door requires at least its own width in clear floor space to swing open, with additional room for someone inside to get around.)

If the space is too small, consider a pocket door, which allows you to keep the door open without blocking the hallway. Specially designed fixtures such as small pedestal, corner, and wall-mounted sinks, and round rather than oval toilets can also conserve space. Because it's the bathroom most often used by guests, a half-bath is a chance to showcase style without sacrificing functionality. "It's typically where you'd want the most architectural detail", says Chris Dallmus, a Cambridge, Massachusetts, architect and a frequent consultant on This Old House projects. "A powder room is an opportunity to do something that's small and intimate but at the same time has sex appeal."