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.
Most apartments are on one level, which is why they are sometimes referred to as a "flat". An apartment on more than one level with its own internal staircase is often referred to as a "Duplex" - many penthouses are designed along these lines. The use of the term is derived from converting two separate units into one by connecting them with an internal private stair. Two story units in new construction are also sometimes referred to as "townhouses". Otherwise, "duplex" refers to two separate units with a common demising wall or floor-ceiling assembly. Groups of more than two units have corresponding names (Triplex, etc.).