Not sure what you mean. There are 3 floors (as europeans define a floor, ie. anything ABOVE what americans call ground floor) and are named floor 1, floor 2 and floor 3.

So, no 0-based indexing here.

We call the house a "3-storey house" not because we zero index, but because the cardinality of it's floors (as europeans define a floor) is 3. In "3-storey building" for Europeans, 3 is the "length", not the last index.