Build a terraced end house with an additional unit (GÜ) on your own

The only thing that would be ill-considered here is if you just run to the [Bauamt]. Besides that, you have to sit 3-4 meters further away on the terrace later. Nothing has happened yet, so you can still talk to each other calmly.

Maybe it was just a mistake.

The only thing that would be thoughtless here is if you just run to the building authority. Besides that, you have to sit 3-4m further away on the terrace later.

Here in Baden-Württemberg, there is the notification procedure (simplified building application). That costs about €200. Neighbors are informed when construction is planned next door. Afterwards, they have two weeks to either approve the application or file an objection. If I do nothing, the building application is considered approved by us.

Therefore, I must formally file an objection. Of course, only if our general contractor also thinks that something is wrong there. Our neighbor has already known about our concerns regarding his building application since last week. So he will not be surprised by this.

Just call him and talk to him about the depth, wait for his reaction and then respond. I think he will still build it even if you disagree; the architect will be liable for the flawless execution with his green dot. You won’t delay or prevent his construction with this as it is normal. However, you can document it yourself preferably, or by an expert, and confront him with it in case of consequential damages. But it could also be that nothing happens and you spoil your neighborhood with your reaction.

Ps. Settlement cracks occur in basically every new building.

Because then our foundation would have to be underpinned, which on the one hand is expensive (for the neighbor) and in case of improper execution can lead to our house subsiding at this point.

...and nothing happens if executed properly?

Just call him and talk to him about the depth, wait for his reaction and then respond. I think he will build it like that anyway even if you contradict him; the architect will be liable for the flawless execution, with his green dot. You will not delay or prevent his construction by this since it's normal. However, you can document it yourself preferably or have an expert do it and confront him with it in case of consequential damages. But it could also be that nothing happens and with your reaction you poison your neighborhood.

Ps. Settlement cracks occur in basically every new building

I don't want to delay his construction at all. We have already had contact many times and he has always told me clearly that he does not want to underpin because it would mean additional costs of 10-15,000 € for him. Just last week we had contact about this topic when we received the letter from the building authority.

Our formal objection would rather be an indication of the architect’s planning error.

The problem is simply that he fully relies on his architect. When I addressed the topic of underpinning and foundation depth in November, his answer was: "That is not his problem but the mason’s."

The good man is still very young, inexperienced, and perhaps a bit gullible and thinks that without verification he can just sign off on everything from the architect, which is now not the case.

Because then our foundation must be underpinned, which on the one hand is expensive (for the neighbor) and can, if done improperly, cause our house to settle at this point.

Well, the costs are his business if it's worth it to him. And the other thing is not necessarily a given! I understand your concerns, but if he wants to build like that, you will hardly be able to change/prevent it and by objecting you might even damage the relationship. If it is done correctly, then it's not really a problem, and "assuming" it wouldn't be done correctly is rather difficult.

Ok, I just saw your further post, then get in touch again and ask once more if he is aware that with the current plan underpinning is required!?