Here in the UK, street level is called the Ground Floor. Pretty neat, since it's on the ground...
Yes, I know YOU know that too. But wait ...
Then it goes pear-shaped...
In England, the First Floor is the one that is up one flight of stairs, so OK, I concede it, you're right, it's not REALLY the first floor.
The floor they built first is the first floor. I know. I can see that.
Except in England, where the floor they build first is the ground floor and the first floor is built second.
Clear? I hope so.
Ground floor? On the ground.
First floor? Built second, on top of the Ground floor.
Anyway, that's how it is, here in Great Britain ...