Some useful words:

• Roof
• Top floor
• The upper floors (in relation to your floor or some cutoff line)
• Your floor
• The lower floors (in relation to yours)
• Bottom floor (could be street level, ground level, or ground floor)
• Basement

Examples:

• I live on the second floor and he on the third floor. He lives on the floor above mine.
• I live on the second floor and he on the first. He lives on the floor below mine.

Exemplification:

My building has 18 floors and I live on the ninth floor. Mr Smith lives on the 18th, the top floor. Mr Jones lives in the upper floors on the 15th. Mrs Jackson and her kids live in the lower floors on the third. The bottom floor at street level is where the building manager lives. The basement is where the water boilers and the washing machines are located.

Source: https://www.quora.com