Floor plan - 135 sqm, 1.5 storeys, gable roof

No, that's not necessarily the case. Nobody said that either.

Oh really?

So if I increase the area on the ground floor to 90, the net/floor area on the upper floor is also 90 ...

Yes, that’s the question whether he can do it like that or not... If I signed for 135 WNFL, then I signed for 135 WNFL. Now the only argument can be about who interprets the abbreviation "WNFL" correctly.

Yes, if you have already signed, just take a look at the contract! What does it say there? If nothing exact is stated, in my opinion the DIN standard for living area calculation applies. Therefore the floor area on the ground floor would be larger, since upstairs the sloped ceilings are not fully or at all included in the calculation.

Correct, 24x25 approx.

Fine!
After 15 pages (on every page you are asked what the measurements are),
Comes a "correct."

I looked at your BT plan again: 31cm thick (narrow) exterior walls are being installed... just for your information... I actually thought that nothing less than 36 is possible anymore, even with timber frame construction...
Well, just a note.
Otherwise, I'll wait and see what comes next.

Net floor area is NOT the exterior dimensions (yours are 10.05x8.2=80.6 sqm and that is different from 67.75 sqm)

Difference between net floor area and living area:
Net floor area also includes areas with less than 1 m standing height or areas where shafts have to be installed. Those do not count as living area.

And, I don’t think you have a contract with a property developer. A property developer sells land and finished house together.

Do you have to go for 1.5 stories? A single level with a gable roof works too – right?

It can't be that you get a single-story house (with knee wall) for the price of a two-story city villa, with the same footprint of the house? At least that's how I understood it in all the discussions?

So, if the net floor area is really agreed upon for the house contract, then that is indeed strange. And then one should really reconsider the possibilities of building [1-geschossig].

It can't be that you get a single-story house (with knee wall) for the price of a two-story urban villa with the same floor area of the house? At least that's how I understood it in all the discussions?

I assume that a two-story townhouse with 135m² of net area was signed for. Usually, the contract states something like "see sketch/plan," from which the two-story design is evident. Since this is not possible on the building plot, the contract could probably be challenged. The simplest solution would certainly be a corresponding change with a new offer from the developer. It is also possible that 135m² of living space was actually signed for and the original poster just mixed up the terminology.