Home of multiple occupancy

There's a few in my street in fact my neighbours house has been turned into one. It was an absolute nightmare during the renovation but I can't complain about them as neighbours, very quiet. My point is there's a terraced house across from me that's being turned into one, I parked outside it today and had a nosey through the window. Oh my gosh it's so small, you'd only fit a single bed and a chair in the room. Surely they should be a certain size?