Dans un bâtiment, l'étage est en élévation au-dessus du rez-de-chaussée et ne peut le désigner.
I would translate it by “In a building, ‘étages” are elevated above the ground floor, which they cannot designate.” I.e., an “étage” is above the ground floor but the ground floor is not an “étage”. You could translate ”étage” by “floor”, but they aren’t exactly the same thing and it would make little sense to by that the ground floor is not a floor.
The term “storey” might be more accurate to translate “étage” as it excludes the ground floor, although it seems it is sometime used to designate basements, which are not called “étages” in french.
The “premier étage” in particular is not what the american call “first floor“.
Architects often use the word “niveau” (“level”), which include both “étages” and “rez-de-chaussé” (and “sous-sols”, “entresol“, “rez-de-jardin“, etc.).
Un triplex est un appartement qui s’étend sur trois niveaux.
Because one of those levels could be the ground floor, you shouldn’t use “étages” here.
So in short: an “étage” is a floor which is above the ground floor excluding the ground floor. It is, in particular, not a basement.