Flat

A self-contained housing unit that occupies only part of a building, referred to in America as an "apartment", and in the rest of the world as a flat. A 'flat' can sometimes refer to any rented accommodation, whether house, part-house or apartment, especially when it is inhabited by a group of young people rather than a family.
You probably already know the BBC series The Flatmates.
See this funny video on how to test the proper noise isolation when you plan to rent a flat.