Recently there have been some questions here about two-storeys, etc. I have always been under the impression that in Europe, the ground floor is the ground floor, and the one above that is the first floor. (Or storey.) Is that true?

If that's so, and you say "it's a one-storey" building - it has only a ground floor/storey? But if it's "two-storey building" it has the ground storey and the first storey? Like, would you say "His flat is the entire first storey of a two-storey building" to mean to he lives upstairs? I'm sorry to sound like a dumb American here, but this has always puzzled me.

And once and for all, is it a three-storied house or a three-storey house or a three-storeyed house?

And do Brits use the word "floors" at all to refer to the levels of a building? (Not that thing we stand on.)
Yes, in Europe the ground floor is the ground floor and the one above is the first floor. ...This does mean that the poor American businessman may have a confusing time here (also here in Holland where we have the same 'floor counting system' as in Britain). Having been invited to attend a meeting with a European colleague "...in my office on the third floor..." (AmE = 4th floor) the American visitor may never be seen again!!

Yes, a "one-story building" (or "one-story...") has only one level, just the ground floor, and "a two-storey building" has the ground floor and the first floor.

Yes, I think you could say "His flat is the entire first storey (or "floor") of a two-storey building" - meaning he lives upstairs.

I think it can be either "a three-storied house" or "a three-storey (or "story") house".

And yes we use the word "floors" to refer to the levels of a building.
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So I know our teaching buildings adopte the so called "European floor counting system", i.e. before we see the signs of “1st floor”, we have to climb up two flights first. The ground floors are just parking lots (I don't know the exact English name for these large lots without doors but with short slopes at the entrances) full of students' bicycles and senior teachers' cars.
It's easy:

When using 'storey': the first storey is at ground level, and second storey is the one above.

When using 'floor': the ground flor is at ground level, and the first floor is abve that.

So the ground floor is the first storey (never 'story') and the first floor the second storey.

It'squite simle really! Why do Americans always want to complicate things? They do have their uses I suppose.
My point exactly
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