noun
BAD : They live on the second storey.
GOOD : They live on the second floor.
◆◆◆
Storey is usually used to describe the structure of a building: 'These office blocks are usually three or four storeys high.' 'a multi-storey car park', 'a detached two-storey house'.
When you are talking about where someone lives/works/goes etc, use floor : 'My flat is on the seventh floor.' 'We took the lift up to the third floor.'