According to various estate agents, I live in an apartment but that's not what I call it. I call it a flat. To me, apartment suggests at least two bedrooms, generous size, possibly split level and some sort of terrace or balcony. Perhaps basement garages and a concierge service in swanky blocks. My home is on the small side, one bedroom, one floor and only communal outdoor space. Admittedly, the kitchen, living room, bathroom and bedroom are all separate. Again, none of the rooms are big.
I wonder if apartment has become estate agent speak for flat, because it helps to sell them ? Perhaps it's one of those American words that's becoming adopted in the UK.