I went to see a first floor flat in a house conversion and it had a sunken bath. I couldn't figure it out why it wasn't sticking out into of the ceiling of the ground floor flat until my friend, a building services engineer, pointed out it must be in the old stairwell. And that there are quite stringent regulations for capping off a stairwell that don't include "put a bath in it". And that the access to the underside, in case of leaks, would likely involve demolishing the bricked up bottom of the stairs, in someone else's flat. It went on and off the market a few times (presumably going back on when the survey dropped and the prospective buyer ran a mile) over a year but someone has now bought it.