https://en.wikipedia.org/wiki/Storey#Numbering
https://en.wikipedia.org/wiki/File:Countries_numbering_floors.svg
Floor numbering is the numbering scheme used for a building's floors. There are two major schemes in use across the world. In one system, used in the majority of European countries, the ground floor is the floor at ground level, frequently having no number, and identified by a word or letter, varying by language (e.g. "G" for 'Ground' in English), or "0" (zero) in an elevator that has strictly numbered buttons. The next floor up is assigned the number 1 and is the first floor (first elevation), the first basement level gets '-1', and so on. The other system, used in such countries as the United States, Canada, China, Japan, Russia, and other ex-Soviet states, counts the bottom floor as the first floor, the next floor up as the second floor, and so on.In both systems, the numbering of higher floors continues sequentially as one goes up,
Height relative to ground (Floors) European scheme North American schemes
Top floor Penthouse (PH), Roof (R/RF),
2 levels above ground floor 2nd floor (2/2F) 3rd floor (3/3F)
1 level above ground floor 1st floor (1/1F) 2nd floor (2/2F)
Partially above ground floor Upper ground (UG), Upper level (UL), Mezzanine (M), etc.
Ground Floor Ground floor (G/GF), Lobby (L), Street (S)
0th floor (0/0F) 1st floor (1/1F)
Partially below ground floor Lower ground (LG), Lower level (LL), Concourse (C), Parking (P), etc.
1 level below ground floor 1st basement (?1/?1F/B1)
2 levels below ground floor 2nd basement (?2/?2F/B2)