In the U.K., Ireland, and South Africa, the first floor of a building is the floor above the ground-level floor. One walks up one flight of stairs to get there. In the United States, Canada, Australia, and New Zealand, the first floor is the ground-level floor. One doesn’t have to walk up stairs to get there. The floor above the ground-level floor is the second floor.
Mathematical logic- First floor/ground level = 0
One floor up = +1
One floor down = -1
EASY
Descriptive terms like basement, mezzanine, floor 4, sub-basement, all rubbish!