So for my sins I work in property in the UK boring stuff, blocks of flats (apartments for the Americans here) leasehold law etc etc and can say it’s subjective.
going into the property from ground level you are on the ground floor, go up the first flight of stairs you are on the first floor, property has a leak, I could guess you may well be in the basement flat. Oh no your car has broken down in the underground parking area (note not basement it’s not lived in)
Also
@Benfro a lot of lifts (elevators) in the will have a GF button rather than a 0 which are adapted for the UK market, some even have a B for the basement
Now where it gets subjective is, if I have to go up a small set of steps to get to the property am I on the first floor or the ground floor depends who you ask and where they are from. Of course we could go into the whole building regulation thing but that’s even more boring!
Here is another fun fact UK fire rescue services don’t count basements or below ground levels as proper levels so the building storey height begins from the ground floor, even though statistically firefighters are more likely to die in a basement fire.
anyway tl:dr if your in the UK it’s ground floor, if you say first floor and you are stuck in a lift you may well get some annoyed firefighters that have humped equipment up the stairs to get your ass out