Neighbor is building a retaining wall on my property. What should I do?

Thank you for the response. This solution is a bit too risky for me. I will try to notify the city in advance about the height of the retaining wall. Let's see what the building inspector says.

It's the same, only that you are doing it covertly and not openly. For me, the relationship is messed up like this anyway. Or what do you hope to achieve with this approach? That the neighbor thinks the authorities took action themselves? He will pay €150 and that's that.

That the neighbor thinks the authority took action on its own?

Since someone is always being blamed, the neighbor will surely know from which quarter the reprimand came.

Or what do you hope to achieve with this approach?

I think this question is a good starting point to even approach the matter here.
If I already spent a few evenings and beers with this neighbor, and he accidentally created a straight line, then I would say "to hell with the 13 sqm" or settle it with a one-time payment.

Apparently, this neighbor has already overstepped the mark several times, and someone has already said here: it keeps going like this, the lack of respect towards you and the moral transgression.

Therefore: show your teeth and sharpen the knife before he overwhelms you!

How is a radius displayed in the cadastre for better understanding?

It varies. Sometimes there are two points and the radius is written on the survey drawing, sometimes three points on the arc, sometimes the center of the circle is also coordinated, and since the introduction of ALKIS in NRW, arcs are divided into sufficiently small chords, because otherwise the decimal places cause errors when rounding.

This solution is too risky for me.

That is not a solution. That is an administrative offense, if not even a criminal act. And even if you were willing to commit it, what would that achieve?

Exactly, the boundary stones actually have nothing to do with it; they always stay where they are whether you have a curve in it or not. In your case, the center point of the curve is shown in the plan. Then mark it on site and set the retaining wall anew.

Since someone is always blamed, the neighbor will surely know from which direction the criticism came.

I think this question is a good approach to even tackle the issue here.
If I have already spent a few evenings and beers with this neighbor, and he accidentally created a straight line, then I would say "to hell with the 13 sqm" or settle it with a one-time payment.

Apparently, the neighbor has already crossed the line several times, and someone has already said here: this will always go on like this, the lack of respect towards you and the moral transgression.

Therefore: show your teeth and sharpen the knife before he overwhelms you!

Especially since at the office only the raised boundary stones are addressed. That will result in a small penalty and that’s it. The boundary issue is settled.

Can someone quickly tell me which federal state we are in?? Because of [Landesbauordnung]??