When talking about a building that has only ground floor, we say [bâtiment, maison, appartement] de plain-pied. Ground floor in France refers to floor zero.
When we say that a building has n floors, it’s the number of floors without ground floor (so it would be n+1 in other languages).

You have it right:
according to the TLFi, étage seems to refer to the space between two floors, so you’d need indeed two floors to make un étage.

A house with only the ground floor has “aucun étage”, it is “de plain-pied“.
A house with three floors, so the rez-de-chaussée + le premier étage + le deuxième étage has deux étages. The ground floor is implicit. This same house has trois niveaux (levels, floors).

Note that historically, both versions seem to be correct:

Le premier étage, anciennt. et encore au Canada, le rez-de-chaussée ;
auj., l’étage carré situé au-dessus du rez-de-chaussée ou de
l’entresol.

Disclaimer after reading some comments: the following statement is based on a few buildings I saw, which doesn’t mean it’s a rule, just that it can happen.
In some buildings and industrial constructions, the actual numbering may vary (probably depending on how it was designed). I have seen the rez-de-chaussée being numbered 1 and the following floors 2,3,4, etc. It’s absolutely not the most common situation, though.

En plus d’étages, on peut parler de niveaux: un immeuble de cinq étages a six niveaux.

the way for anglophone to solidify a ‘floor, or, ‘space of each floor’ as étage is think of ‘a stage.’ Each level is a stage that life is performed.