From which parapet height is considered a full floor?

Would a balcony on the upper floor create leeway to raise the knee wall further? Or are there other tricks that allow raising the knee wall?

if the knee wall is more important to you than the area: build a setback floor, i.e. with a recess, possibly with a roof terrace. Or a balcony that stands on the floor area of the ceiling, not a "hanging" one. Assuming you take out 1/4 of the area as a roof terrace, then you can easily build a knee wall of 200 cm or more and then close it off with a 30° roof pitch. If you want to get the maximum out of it, I would definitely see an architect for the planning.

That may be because it depends on the floor area of the new building and cannot be answered generally. For you, it’s not 2/3 (as in other countries), but 3/4… You can roughly determine that yourself with graph paper and a set square. The architect you trust can tell you definitively… Impossible for a forum. North Rhine-Westphalia Building Code NRW, § 2 Terms, para. 5, 6

the building code from the year of the development plan applies, so 1962. Is that the version?

Would a balcony on the upper floor create leeway to raise the knee wall further? Or are there other tricks that allow you to increase the knee wall height?

You don’t need that, and it would rather be a self-goal to wish for it. With the search term "Fensterscheide" you will find among other things my post here in and externally (including the quotation marks to google) I have also discussed the topic in detail in "How the knee wall influences the window question in the attic". You don’t put a broom closet in every corner, so you don’t need a maximum knee wall (and knee walls – see "Bau-Wörterbuch" – are not limited at all). What I can’t link here because of the sheer quantity are the posts in this forum dealing with the abundance of the most outrageous tricks for pushing up the knee wall height, bulging out the attic space and the like. In my opinion, the only sensible cure against knee wall maximization mania is my aforementioned external post ;-)

Would a balcony on the upper floor create room to raise the knee wall further?

Why do you want to raise the knee wall? It reads as if you have an offer with a specified knee wall. Then please say what this is about. The point is surely not to plan a balcony instead of living space just to be able to raise the knee wall by 20cm.