Request for knee height increase +20cm

you probably mean the drawings for the building application

Yes, that's what I meant : )

Is there anyone in the neighborhood who has the meter? Then ask them.

No, there isn't, but it's a small community, they shouldn't be too busy now. I think we'll give it a try : )

However, I could imagine that you would then need completely new plans from the architect... What does he say about that?

We have only submitted one building notification and had our special request - deviation from the building envelope - approved in advance through a separate application (~100EUR).

You need an architect... What does he say about it?

I don’t have an architect, only a draftsman. And he still has to be asked : )

or will it be noted at the knee wall that it may only be 80 not 100cm high, and then the development plan is approved?

I rather don't believe that. It also slightly changes volumes / heating demand calculations. And usually people would want to adjust the roof pitch instead of getting 20 cm less room height linearly in the sloped area.

So I just don't want to waste time unnecessarily with multiple rounds.

Then I would call first: for a "yes" this way is probably not binding enough, but a "no" in the sense of "this is about principle, we define the Franconian character of the building appearance by the parameter midi knee wall height" should also be clearly communicated verbally.

The latter is a common motive to fix such knee wall heights – especially where the ridge height is not so stingy, so it's apparently less about the building volume and primarily about the knee wall itself, which is to be limited.

Then this limitation is gladly regarded as a central architectural folklore factor ;-)