I understand that the word basement means:
the part of a building that is wholly or partly below ground level
[Merriam-Webster]
But I wonder what American English and British English call 'the first basement floor right below the first floor of a building' and 'the second basement floor beneath the first basement floor'.
Is it all right to call it 'basement one' and 'basement two' in both AmE and BrE?
The most common English translation for what the Chinese call this type of floor is underground floor, or first/second/third/etc. floor underground, as evidenced by this link: http://eng.ichacha.net/underground%20floor.html
Is this terminology also acceptable in colloquial AmE and BrE, or is it more technical in nature?
Best Answer
The answer to a "proper" expression for underground floors is that there is no standardized nomenclature, rather, there are multiple different designators depending on the place in question and the country: https://en.wikipedia.org/wiki/Storey#Subterranean_floors
Of course, that's not to say that people won't understand you if you use first floor underground, but I think the reason this isn't widely used is because it's just a mouthful. Five syllables, versus the three syllables of basement 1, or first basement.
First floor underground has been used, though, and can be found in various engineering books, particularly in the English translation of Chinese manuals:
Code for Fire Protection Design of Buildings
The 2nd International Symposium on Rail Transit Comprehensive Development ...
Advances in Civil Engineering and Building Materials
Rainbow Town
Civil Engineering in Japan, Volumes 6-8
CCDI Architecture: Interaction with a Complex Context
Re/collecting Early Asian America: Essays in Cultural History
...and more can be found in Google Books
Ngrams shows that basement floor is the outright winner compared to underground floor/floor underground/basement level/floor basement/lower ground floor/lower lobby/lower level floor/lower floor level/sub-basement/subbasement/sub basement. I didn't include lower level because the results would be corrupted by the common definition of level.