The post of our driveway gate protrudes about 10 cm onto the neighboring property

Just because I park my vehicle illegally on your private parking space doesn't mean it belongs to you or is under your control.

The fence is firmly attached to the property, the vehicle only if it has been there for a very long time. But the answer from makes sense.

Correct.

We agree, except that for professional reasons I advise against DIY; how else am I supposed to feed my children ... Joking aside, because finding boundary points is no fun and, in case of a dispute involving 10cm, also not helpful.

Especially because the stones are not always placed exactly where the boundary point is. Sometimes the stone is exactly 1 meter away because something was in the way or something like that. This is visible in the documents, but not on the stone itself.
With the new points for my subdivision, this is now the case. One stone should have been placed directly under the still existing asbestos corrugated roof, but the surveyor probably didn't dare to do that...
Another one is on the overgrown neighboring property where no owner can be found. So both stones lie 1 meter away from their actual point.
So could it even be possible that something like this happened here? A supposed stone was discovered on your property. The neighbor or architect assumes the wall is on his property, but in reality the wall is exactly on the boundary and the stone was therefore buried in a shifted position...

...and just as there are countless examples where the boundary stones are exactly where they should be and have been all along.

...and there are stones which were originally placed incorrectly, etc. etc.

...and there are also property owners who remove the stones completely.

and now?

But, let's put aside this constant pessimism and doom-mongering... if the stones are now suddenly exactly where they should be (and the chance that they are exactly there is usually higher than the other way around), it is really the easiest way for the OP to set everything straight without spending a lot of money.

..if the stones are suddenly exactly where they should be

And how is he supposed to assess this as a layperson? In the end, he believes he is right, makes a discount, and the neighbor is actually right. Or vice versa, there is a stone on his side, he believes the neighbor is right, but in reality, he even has 10 cm more. Then he is giving away land. In my opinion, a professional is definitely needed here.

So first of all, the question is whether the original poster has to prove that the post is on his property or whether the new neighbor has to prove that the boundary runs differently? It is basically a question of how official and accurate the submitted documents are and what exactly they state. If boundary stones are found, it may be possible to draw conclusions, but they do not eliminate doubts.

If the neighbor/architect bases their opinion on their view, one will not avoid a boundary determination.