A powder room is governed by clearance arithmetic long before it is governed by taste. The usual working figure is around 380mm from the toilet centreline to a finished wall each side, with roughly 600mm of clear floor in front of the bowl. Less is buildable and reads as tight. A basin wants its own clearance, and at this size the two overlap, which is why a comfortable two-piece tends to start near 900mm wide by 1500mm long.
The door is the biggest object in the plan. A 760mm leaf swinging inward sweeps an arc that a toilet and a basin cannot share, so in a narrow room an inward door and the fixtures compete for the same wall. Swinging outward solves it where the hallway is wide enough and the open leaf does not block a route. A narrower leaf helps. A pocket door works well here in a way it does not beside a bedroom, because there is no sleeper on the other side — though a powder room off a dining room is exactly where the missing seal is noticed. A barn-style slider needs clear wall beside the opening and seals nothing at all.
Under a stair, head height is the deciding measurement, and it is measured where someone stands rather than averaged. Clear height above the front of the bowl matters most, because that is where a person straightens up. The toilet goes to the tall end, a wall-hung basin fits under a rake that would defeat a pedestal, and a concealed cistern in a stud frame trades 200mm of depth for usable floor.
With no window, ventilation is mechanical and it has to reach outside; a recirculating charcoal unit filters air and ventilates nothing. The load is odour rather than steam, so the fan is chosen for quietness and run-on rather than raw airflow. The detail that decides whether it works is make-up air — a tightly sealed door in a sealed little room starves the fan, and a 10mm undercut or a transfer grille is what lets it actually move anything.