Reconstruction by neighbors along the property boundary

Let's summarize this.
The neighbor is planning an extension and asks you if that's okay. You say yes. Then construction starts and you think, "Oh dear, that's going to be big, I don't want that anymore!" That's not how it works, you can't just withdraw your approval after a certain revocation period.

By the way, it is also said right away that the new solution is better than the old one, because the old shed didn't even have a gutter. That's a pretty dangerous statement if it goes to court. Because apparently, no one was bothered by the rainwater from the old shed for decades, and the neighbor can't do anything about increased heavy rainfall.

Now you went to a lawyer who says, "Okay, everything seems fine, I don't see any leverage, except for one trick."

And you want to use that trick now, hoping that the neighbor will tear down their expensive built extension and rebuild the old wooden shed without the gutter? Because apparently that was fine all those years.

I suspect a court in such a case will first come up with the wider gutter, whether that helps or not.
And in the end, the question is also, did the water only come from the neighbor's roof? Why doesn't it drain on your side?
And so on.
You imagine something like this, especially in court, way too simply... these things can drag on for years and almost always end with a compromise in the end. You can only prevent such extensions if you file a lawsuit before they are built.

PS:
And because it was portrayed a bit contemptuously that he apparently just wants to have peace at last. I think he has better things to do than to bother with the disputes of old stubborn people.

You don’t really want to make an inheritance matter out of this, just so your mother isn’t so upset? That’s childish. Your mother should stand up straight, adjust her crown, keep her dignity, and leave the sensitivities alone. I understand everything, but at some point you have to show some backbone – after all, she wants to live in her property and not pass it on. That has nothing to do with the whole thing anymore. For me, giving up the property would also be a complete step backward in terms of the neighborhood dispute.

Well, the mistake was actually agreeing to this gutter or the overhang on the own property. This can only lead to problems in the case of heavy rain, when the sewer system or the cistern, soakaway pit, etc. are overloaded. Even a larger gutter won't help in extreme weather because the "blockage" or saturation is somewhere else.

But all that doesn't help anymore, because the mistake was made and the revocation was delayed too long. Now the only thing left is to talk reasonably with the neighbor, point out the damage caused, and try to find a solution when 50-100l/m² falls in a few hours again.

And part of the solution, besides the larger gutter, could initially be to store the firewood elevated and not on the ground, where the water stands for several hours before it drains.

I also have a neighbor here who wanted to finish his driveway long before me and therefore placed the concrete wedge on my property. We still get along well, but now I have to deal with it somehow. Because he can no longer simply remove it, then his curbstones would tip over and the paving would "flow away." I could call him an idiot every day now, but that doesn't help. And a court would again seek a compromise in such a case and not demand the complete removal of the driveway. That's just how it is. Sometimes bluntness is (often) trump.

I find the fact that there is the same story with reversed roles in the Blue much more interesting. There, the OP built at the border and the neighbor complains and wants to give the house to the daughter. Very dubious!

I do not deny that at all and have thereby also understood how both sides must currently feel. When double posting, I truly had no ill intentions and just wanted to gather information faster to better understand the basic facts for both parties without having to go to a lawyer or even risk a court. But she is so stubborn, probably because she is a Taurus, and simply will not be appeased. I just have to call the lawyer again, hope that he can also speak out regarding the inheritance and the claim made by her through me afterward, and apparently also the newly associated property right, because of which the finished gutter overhang along with the found and verbally agreed compromise will likely go against us, because there apparently aren’t two winners in the matter, as I originally intended, since on the one hand I fully support my mother, but on the other hand also did not want to risk the now apparently ruined friendship with them. We’ll see how it turns out, who loses and who will be mad at me forever. Thanks for everything and have a nice weekend.

I just have to call the lawyer again,

I have never understood from the beginning why you are digging around in there like that. Your mother is obviously mentally fit enough and now wants to change something that she previously arranged herself. If she wants such a dispute, then she can carry it out alone and you can at most drive her to the lawyer. You can be loyal to your mother without carrying along every nonsense. Here, a "no" from the adult son to the equally adult mother can also be helpful. You are not just any errand boy for your mother’s wishes unless you want to be. Now you were at the lawyer and in the end, neither of you knows the facts; apparently, you are not the right person at her side, and I do not mean this in a bad way at all, because such conflicts are simply not everyone’s thing. Why can a son not simply say no?

because on the one hand I fully support my mother, but on the other hand I didn’t want to risk the now probably ruined friendship with them

Then just stay out of it.

Let’s see how it turns out, who loses, and who will be angry at me forever. Lean back and the problem will solve itself for you, because it is not yours.

If you stay out of it, there is no reason for any of the older ladies to be angry with you. Unless you finally need to make it clear to your mother that you do not want to have it this way or be part of such a dispute.