For some reason this is one of the points readers of mine (including off-site ones) have mentioned several times, perhaps because it's one of those Transatlantic divides that isn't all that well known. So: when you're talking about the floors of a building:

US English: one story, two stories. The same spelling and the same plural as for the other kind of story.
UK English: one storey, two storeys. Not the same spelling or the same plural as for the other kind of story.

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