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

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

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?

I understood it as meaning that the idea leads to the building authority having to intervene.

Can someone quickly tell me in which federal state we are?? Regarding the state building code??

In BW

In BW

The retaining wall is not covered by a building permit. You wrote that your neighbor was granted 1.20 m. The development plan only allows 1.00 m. So he needed a waiver from the development plan for the permit already?
Whether the wall is or would be permitable cannot be assessed from here. If it is a quite new development plan, I would say rather no.
If you contact the building authority, it may require revised plans to be submitted with the actual height of the wall, or it may order the dismantling of the wall if it is clearly not permitable because no waiver is in sight.
If a subsequent permit comes into question, you as the owner of the adjacent property will be heard and can then raise objections.
Whether the wall then violates neighboring protective rights must be clarified in the procedure. In any case, you can argue that you do not consent to the wall being erected/was erected on your property. Then no building permit can be granted to him due to a lack of legitimate interest in the matter (meaning he obviously cannot make use of his building permit).

Then the culprit was known.

That would be even better.

Building authority is a collective term for all municipal offices involved in construction.
From which office exactly?
With what content?

What exactly was done? Were the boundary points remarketed?

At the surveying appointment? Which federal state?

Are you sure the rough carpenter informed you about everything in this regard?

Then don't get caught (VermKatG NRW, the other federal states have similar regulations):
§ 27 Administrative Offenses
(1) A person commits an administrative offense who
...
4. intentionally or negligently attaches, removes, or alters boundary markers contrary to § 20 paragraph 5,
...
(2) The administrative offense can be punished with a fine of up to fifty thousand EURO in the cases of paragraph 1 numbers 1, 2, and 6, and in the other cases with a fine of up to three thousand EURO.

Who paid then?

To 1) Yes, the rough carpenter
To 2) It just happened while setting the L-stone. We reported it to the managing director on the same day
To 3) Letter from the building authority with a request for a statement + feedback on why markers on the boundary points on the L-stones were carried out by an unauthorized party. The statement came from the rough carpenter
To 4) Boundary points and boundary (i.e. my L-stones) were re-surveyed. Markers matched in the west. In the south not (-4cm), therefore not built over. Hence a new marker (pole) in front of the L-stones.
To 5) Yes, all present at the surveying appointment or represented. Bavaria
To 6) Yes, he did
To 7) The invoice went to the rough carpenter. But that does not mean he didn’t pass the cost on somewhere else.

Otherwise, the idea is of course not legal. But going over the height of the L-stones will only work if the building authority reacts sensitively to such things. If not, there is probably only a subsequent exemption + possibly a fine.

The OP can also report to the building authority that the boundary is no longer correct and listen to the building authority’s proposed solution.

Bayern

In Bavaria, many things, especially in surveying, are different from the rest of the republic. Thank you very much for the detailed description. The fact that there were no disputes was due to the sincere actions of your shell contractor, who took on both responsibility and costs.

I understood it as the idea leading to the building authority having to intervene.

But that is not the case.

The retaining wall is not covered by a building permit.

It doesn't have to be, since retaining walls in BW up to 2m in height are exempt from permit requirements.

or it orders the dismantling of the wall

It can only do so to the extent that the height exceeds the development plan requirement. If the original poster is lucky, they will find an ally at the authority, so that the public-law demands also remedy the private-law violations. From a purely legal point of view, the authority has nothing to do with the encroachment.

It doesn't have to, since retaining walls up to 2m in height are exempt from approval in BW.

But not if the development plan only allows walls up to 1.00m.
Exactly then, the wall is not approved either formally or materially, nor is it eligible for approval.

Since no formal approval is required, I do not see your approach through neighbor involvement and lack of factual decision interest. The TE should undoubtedly contact the authority, but should not have overly high expectations. The actual problem of the boundary encroachment cannot be resolved by the authority. The detour via the excessive height at best gives hope for an indefinite removal order, which leads to a complete demolition.