first floor; ground floor
In Japanese building numbering, 一階 normally means the street-level floor. This corresponds to 'first floor' in American English and often 'ground floor' in British English.
The reception desk is on the first floor.
Let's meet at the cafe on the ground floor.
one floor; one storey
Uses 階 as a count of floors or storeys, often in compounds or measure-like phrases such as 一階建て and 一階分.
This house is one storey.
I carried the luggage up one floor's worth of stairs.
Rare abstract use of 階 as a rank or grade. In everyday modern Japanese, expressions such as 一つ上, 一段上, or 一級上 are usually more natural.
The expression 一階上の位 means something like 'the rank one level above.'
There's a convenience store on the ground floor of this building.
My grandmother lives on the ground floor.
I took the elevator down to the ground floor.
The new house is bigger by the space of one floor.