The maximum height of the knee wall, everyone says something different.

No, the floor area ratio alone does not impose the restriction, the site coverage ratio and floor area ratio of 0.3 each are merely maximum values. If it were only based on these, a "city villa" would be possible with an actually utilized site coverage ratio of 0.15. There is no eaves height restriction here, so the only thing standing against 9.00 m building height is the "I" full floor, since a 30° gable roof could possibly start above the upper floor ceiling.
According to your image excerpt, the case is clear: the reference height is 7.99 meters, and thus the base height is a maximum of 8.49 meters and the ridge height at 16.99 meters. Height-wise, I see the biggest restriction here in the non-full floor. My bigger concern would be the trees and the two parking spaces.

Thanks for your super quick reply!!!!

As I understand you, a full floor would be possible due to the site coverage ratio and floor area ratio and the absence of an eaves height restriction, but not because of the "I". Accordingly, one could realize a solid knee wall, possibly 1.20 meters or more? What do you think?

What would be your concerns regarding the park and the trees? The wet area can of course cause quite a few mosquitoes in summer after rain. But otherwise, you are practically well situated in the green. The park ends there because there is a field to the west after the properties. So there should not be too many people there. Or do you mean the trees might cast too much shade? For the leaves I will get a monster leaf vacuum :-D

90cm (solid house provider) and 1.20m prefabricated house provider. Is it due to the construction method or the inexperience of the salespeople?

In my opinion, it is because several different factors make a single-story house a single-story house, e.g. roof pitch, dormers… initial general estimates then lead to different statements.

the question is simply, what can we effectively get out of the 1st floor.

Don’t fixate only on the knee wall. The living value and quality of life of a house also depend on other factors.

For SH applies

A top floor set back from at least one exterior wall of the building or a floor with at least one sloped roof surface is a full floor if it has a height of at least 2.30 m over at least three quarters of the floor area of the floor below

It would therefore also be possible to have a stepped floor (setback upper floor) that occupies <75% of the area of the ground floor – and on top of that a gable or shed roof, in which further habitable rooms would again be permissible.

For SH the following applies

A top floor set back from at least one external wall of the building or a floor with at least one sloping roof surface is a full floor if it has a height of at least 2.30 m over at least three quarters of the floor area of the floor below

So a recessed floor (set-back upper floor) that occupies <75% of the ground floor area would also be possible - and on top of that a gable or shed roof with further habitable rooms would be permissible.

The idea of the recessed floor doesn't sound bad at all, is that something for prefabricated house builders or are they out of the picture?

It depends on the builder... probably not the cheaper ones. And changing an existing floor plan costs quite a bit of money. Are you fixed on prefabricated houses, or can it also be a solid builder in the form of a general contractor?

That may not help you directly, but a experience from us: we also wanted to get the maximum knee wall and visited several prefabricated and solid house providers. The statements ranged from 60cm to 1.40m. If you are restricted in height, it depends on many factors. One provider did not want to put anything in the ceiling and worked with 12cm suspended ceilings, which already costs you almost 25cm over two floors. Also, the providers planned different heights for the basement on the slope, that is how deep it was set. One prefabricated house provider had a super thick floor structure, and that was also the one with the 60cm.

In the end, we had some floor plans designed and are now building a solid house with 1.30m knee wall in the upper floor and 2.50m finished height on the ground floor. A good architect gets the most out of what is possible.