structural:expansion
One dwelling with two storeys

The vast majority of apartments are on one level, hence "flat". Some, however, have two storeys, joined internally by stairs, just like many houses. One term for this is "maisonette", as above. Some housing in the United Kingdom, both public and private, was designed as scissor section flats. On a grander level, penthouses may have more than one storey, to emphasise the idea of space and luxury. Two storey units in new construction are sometimes referred to as " townhouses " in some countries (though not usually in Britain).