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?
Not sure yet! In the UK, the term apartment is more usual in professional real estate 
what else can you tell me about this?
Private equity real estate
Private equity real estate is a term used in investment finance to refer to a specific subset of the real estate investment asset class. Private equity real estate refers to one of the four quadrants of the real estate capital markets, which include private equity, private debt, public equity and public debt.
Overview.
Direct vs. Indirect Ownership of Real Property – Private equity real estate investing involves the acquisition, financing and direct ownership and holding of the title to an individual property or portfolios of properties
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