Under stairs bathroom actually makes sense. What’s more, what else could one possibly put in an area with such shallow depths and awkward ceilings?! A bathroom can’t be a bad idea for this otherwise odd space basically found in almost any 2-story house design. Mirrors, showers and sinks will be problematic; on the other hand toilets and tubs don’t require full standing height to work, because they can be sat on or lain in respectively. Small is a bit more acceptable for a bathroom than it would be for a living room or bedroom, because its uses tend to be short. With that in mind, there’re several ways to do it wrong as well; poor lighting will ruin any space and a tiny one all much more quickly.