From Jo's ref, courtesy of wsieber:


The change probably happened when the elevator-makers expanded their business from Great Britain to the European Continent. German "Stock" and French "Etage" intrinsically mean a plane above the ground, so "1.Stock" never meant anything else than the first "elevated" floor.


That makes sense, and explains the difference well.

OK, Jo, so how many floors would Brits say my hypothetical building has, i.e. G,1,2,3,4 (USn G/1,2,3,4,5)?

Check the whited out text below after you've decided!

I think it might be four floors, discounting ground floor