I've worked in real estate on the east coast and have never heard 'flat' used regarding rentals. Ever. Also, as you mentioned, one walled residences can be called 'duplexes' if they're a rental or owned, as well as the term 'semi-attached *whatever the type of house it is* ie; semi-attached townhouse. I wonder if what we in the US call 'garden apartments' are what are called 'terraced apartments' in the UK. EDT: Just checked. Nope. Terraced apartments are one type of high-rise apartment houses in the US in which the external apartments all have a small patio.