Architects use to use 'basement' for the whole lower section of a building, das "Kellergeschoss"
In German they count the following way:
0 Kellergeschoss (souterrain) = basement / cellar, (this is underground or at least half way)
1 Parterre = ground floor/bottom floor,
2 erste Etage = 1st floor,
3 zweite Etage = 2nd floor,
(n) Obergeschoss = top floor, (last full size floor)
(x) unterm Dachjuchhe = attic. (applies to peaked roof buildings)
Base = is related to where the slab/foundation is, it includes the whole lower area of the building.
In Europe a basement it is always fully underground, whereas in Australia a basement can be also half underground. In Europe they would call that a sou-terrain, especially for houses build in a hill or mountain.
A "Keller" -cellar is mostly used for the part of a 'basement' allocated to a flat/person/tenant, or by purpose, like: "Weinkeller" -wine cellar, or historical: "Auerbachs Keller", "Kartoffelkeller", potato cellar. Also restaurants can have the by-name "-keller" especially when they have such an underground floor and are known to serve wine for instance.
But "Tiefgarage" is used for a garage in the basement of a house. You would not call it a "Kellergarage!" :-)
Inconsistency.First it is a cellar and suddenly a basement.This happens frequently with Duolingo
I have never heard of a basement called a "Keller" in Germany, only "Untergeschoß". In English, we would never call the basement "the cellar"; the two terms are simply not interchangeable. The basement is the level of the house that is below the main level. It can be completely or only halfway Underground. It can be a properly finished living space or an unfinished storage area. The cellar is a storage area that is underground, specifically to keep storages of produce, and home-canned foods from harvest, through the year, at a naturally-refrigerated temperature. Even in the rare situation where a person has to traverse the basement to gain entrance to the cellar, the two terms are descrete from one another.