Why is the bathroom light switch outside the room in some countries and inside in others?

Adding the non-regulatory half, because plenty of outside switches are not driven by the rules at all.

Bathrooms are the smallest room in most houses, and by the time you have fitted a door that swings inward, a basin, a towel rail and a mirror, the wall area next to the door on the inside is often already taken or blocked by the open door itself. The wall on the other side of the same doorway is usually empty.

There is also a genuine convenience argument that people forget once they are used to it going the other way: you light the room before you walk into it, rather than groping along a wall in the dark. Hotels that light the bathroom automatically are solving the same problem with a sensor.

So: regulation explains why it is common in some countries, geometry explains why it survives even where it is not required.

There is a real reason, and the pull cord is the clue that ties it together: it is about keeping mains voltage away from wet hands.

European wiring rules divide a bathroom into zones measured from the bath or shower, and each zone limits what may be installed in it. Ordinary switches and sockets are restricted in the zones nearest the water. A small bathroom can easily be almost entirely inside a restricted zone, at which point there is nowhere compliant to put a switch.

That leaves two compliant options, and you have seen both:

• Put the switch outside the door.
• Use a pull cord, where the only thing your wet hand touches is an insulating string and the switch mechanism itself sits in the ceiling, well away from everything.

Countries where the switch is inside the bathroom generally have rules written around a different assumption — usually that a residual current device on the circuit handles the risk, so the switch position matters less.

Small traveller's note: in older buildings, check whether the outside switch also controls the extractor fan, because they are very often wired together on a timer.

If you turn the light off at the wall as you leave, you sometimes cut the fan run-on with it, which is why some bathrooms in old guesthouses never seem to dry out. Leaving it on for a few minutes after a shower does more than any amount of window opening.

The obvious objection, since everybody arrives at it within about a day: yes, somebody can turn the light off on you from the corridor, and yes, this is the single most reliable practical joke in any building where the switches are outside.

Worth knowing that newer installations often avoid the whole argument. An isolated switch with mechanical separation, or a low-voltage switch driving a relay elsewhere, satisfies the same rules while sitting inside the room. So in a recently rewired building you may find the switch has moved back in, next to a bathroom that is otherwise identical to the one down the road.