Permissible wall height - what does that mean?

If I have understood correctly now, the 4.20 m eaves height is measured from the property level (or from the centroid of the house) and not from the sidewalk as height 0, if you can put it that way? This results in the fact that in addition to the ground floor, an attic with knee walls would be allowed. The house is supposed to be rectangular. Fortunately, the centroid is easy to determine.

The building authority has unfortunately been no help to me so far.

A wall with an integrated staircase directly at the street currently bounds the property. However, I fear that it will have to be moved because the development plan specifies a minimum distance of 0.75 m from the street and it is currently built directly on the street. Or is there something here regarding protection of existing structures?

if one can put it that way?

Yes, IMHO you can put it that way.

since the development plan stipulates a minimum distance of 0.75 m to the street

What exactly does it say?

Or is there something like existing rights here?

Unlikely for rather low-value buildings. But your architect should take a closer look at that.

And something like that is actually remeasured?