— Kris in Highland Park
Answer: From licensed general contractor Alon Toker of Mega Builders:
The minimal dimensions of a powder room would be dictated by the elements that are in it.
The door needs to be 32 inches wide. The toilet must have a wall space of 30 inches minimum (15 inches to each side from center of drain). Then, 21 inches are needed in front of the toilet and in front of the sink.
Once all of these minimal requirements are met you will have your minimal room space. To meet these requirements and still keep the room size to a minimum, try a wall-mounted toilet (which is more expensive than a typical floor-mounted version) and/or a corner-mounted sink.
This said, I would advise you to revisit the notion of going to the absolute minimal size possible. Allow yourself a bit more space, if possible, so the bathroom has some visual "breathing room" as well as physical room. For example, 32 inches of wall space for the toilet, rather than the 30 inches required, would provide an adult with a more adequate amount of elbow room.
Photo courtesy of Kitchen and Bath Design News