Knee wall height - designation in the development plan?

Your way to the municipality is good.

Does the same development plan apply on the other side of the street? If yes, the area was divided into different heights.

Maybe the restriction on the knee wall can be waived with the justification of the massive development opposite?

Also ask how the knee wall height is defined in the municipality. As you can see above, this can make a difference of 50 cm in construction height. Every centimeter helps!

Are there developments on your side of the street that might differ from the plan? Then take pictures of them to the municipality.

From my point of view, you can basically still build up 32 cm under a purlin that is 18 cm high here. That will become what I call a "vacuum cleaner knee wall."

"The height of knee walls, measured from the top edge of the ceiling to the intersection of the outer edge of the masonry with the top edge of the purlin, must not exceed 0.5m."

In my opinion, the regulation is too vague insofar as it does not specify whether it refers to rough or finished measurements. But since it speaks of "ceiling" instead of "floor," I assume it means "rough."

Number of full stories: 1 (mandatory)

That is nonsense, as "max. 1 full story" and "1 full story" are practically identical. Theoretically, without this additional "mandatory," a nitpicker could place an attic floor without a ground floor below it *LOL*

What does top edge of the purlin mean? Shouldn't it be top edge of the rafter?

No, purlin is correct. After all, it is not measured up to the top edge of the roof covering here; the drawing in post #2 is unfortunately wrong.

especially on the opposite side of the street there are buildings with 3 full stories....

That does not have to be the same planning section.
You can name the development plan here ("Posemuckel No. 234 in the cool valley," links are not desired here).