Someone asked a question recently about how small a toilet room can be. They said the door didn’t close when you were in and your elbows were touching either side of the walls.
There’s no room size minimum in building regs that I could find for bathroom size – am prepared to be corrected if you have some info though.
There used to be guidelines on minimum house size from the The Parker Morris Committee drew up an influential 1961 report on housing space standards in public housing in the United Kingdom titled Homes for Today and Tomorrow.
This wiki article also says that most new builds today fail to meet these standards!
This site has a nice graph to illustrate the floor space principals – but the most annoying cat ever!
The Mayor of London report on space standards is worth a look too
Maybe we should have a competition to find the smallest studio appartment in the UK?