In some parts of the world, the word apartment refers to a new purpose-built self-contained residential unit in a building, whereas the word flat means a converted self-contained unit in an older building. An industrial, warehouse, or commercial space converted to an apartment or flat is commonly called a loft.

When part of a house is converted for the ostensible use of a landlord’s family member, the unit may be known as an in-law apartment or flat or granny flat, though these (sometimes illegally) created units are often occupied by ordinary renters rather than family members. In Canada these suites are commonly located in the basements of houses and are therefore normally called basement suites.

In Milwaukee vernacular architecture, a “Polish flat” is an existing small house or cottage that has been lifted up to accommodate the creation of a new basement floor housing a separate apartment, then set down again; thus becoming a modest two-story flat.