In the US a "flat" is an apartment that spans an entire floor of a building. In that sense it is the opposite of a townhouse (an adjoined multi-unit dwelling w

In the US a "flat" is an apartment that spans an entire floor of a building. In that sense it is the opposite of a townhouse (an adjoined multi-unit dwelling where the units are vertical instead of horizontal). Some say it is rarely used but there are currently thousands of "flats" available for rent on craigslist/sfbay, so I think it's not that rare.