Isn't a flat precisely that - a flat. If I wanted stairs I would probably buy a house. In a flat the stairs could be seen as “wasted space” in a house the stairs offer the potential of increased square footage inside the house build on a restricted sized parcel of land. I think lateral living is actually quite good for increasing the feeling of space in a building. The place seems bigger if you can, for example, look from one room and see the end of the next room quite a long way away.