apartment or flat (British English) or  is a self-contained housing unit (a type of residential real estate) that occupies only part of a building, generally on a single level
What is it called in the united states?
Its called a flat
Thats awesome 
Scotland, it is called a block of flats or, if it is a traditional sandstone 
Are there regulations concerning this?
in North America (although in some cities "flat" is used for a unit which is part of a house containing two or three units, typically one to a floor). In the UK, the term apartment is more usual in professional real estate and architectural circles where otherwise the term "flat" is used commonly, but not exclusively, for an apartment on a single level (hence a 'flat' apartment).
In some countries the word "unit" is a more general term referring to both
layouts.
An apartment (in American English) or a flat (in British English) is a self-contained housing unit (a type of residential real estate) that occupies only part of a building. Such a building may be called an "apartment building", "apartment house" (in American English), "block of flats", "tower block", "high-rise" or, occasionally "mansion block" (in British English), especially if it consists of many apartments for