Floor area calculation maisonette apartment

Isn't the difference between living area and usable area just the different usage? At least in my old condominium, living area was equal to usable area minus the basement. Even though there are sloping ceilings there. And since it is about areas (two-dimensional) and a house has volume (three-dimensional), you have to make cuts somewhere. The only logical way to make these cuts is at the floor slabs and then only count as an area what also has one. In other words, the answer with the carpet leads exactly to this solution. Then also subtract the sloping ceilings accordingly.

There is a misunderstanding, I did not ask any question about the calculation of living space. It is only about a specific question regarding usable floor area - which indeed becomes complicated when terms are separated contrary to the topic or the calculation of living space is described.
But I am not responsible for that, the question clearly stated what it was actually about.

Yes, I no longer remember your correct question, sorry

In my opinion, the usable floor area is the entire floor area... anything under one meter in height still does not count.
Usable floor area is equal to or greater than living space.

When I look at Wikipedia, the usable floor area for apartments is regulated in the calculation of living space. Have you already checked there?

OK - if I understand correctly, the upper area is counted as additional usable space if there are rooms with partition walls and doors there, and not counted if the upper level is open, without walls and doors?

No, I already said that in #2: it does not matter whether an area is bounded by a parapet or a floor-to-ceiling wall.

I want to sell and provide accurate information.

Then you should not use the imaginary measure "area of the floor covering" (regardless of whether on one or both floors), but rather the information according to DIN or the Wohnflächenverordnung. Both have in common that they fully include areas with full height, partially include areas with half height, and do not include areas with very low height at all; and this method of calculation applies regardless of on which floor the rooms or areas are located, and equally applies to usable areas (e.g. area of the storage room or heating room) as well as living areas (simplified: areas of rooms intended for living).