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

As it looks, his architect planned the basement 30cm deeper than our foundation.

I don't know much about this. Can you explain why this is going to be a problem?

But hopefully only after you have discussed the problem with the future neighbor? It would be more than a bad start... or has the conversation already failed?

To good neighborliness...

The poor builder, who probably relies solely on his [GU]...

I would first talk to the builders about it...

But that would really be the very first step. Immediately seek the conversation, then you can look further.

They build terraced houses and everyone wants to be the biggest individualist.

We have sought good neighborly relations from the very beginning. We provided our neighbor with all documents, including those for our civil engineering work, already in November. At that time, I also sent an email with all the documents and the facts to his architect. If then the submission documents are faulty, it does annoy me a bit – and when I hear from the neighbor that he thinks his architect had taken that into account, I also lack some understanding.

We have already been in contact about the matter these days. Furthermore, I must file an objection as a precaution because there is a two-week deadline to be met. Then his architect should simply plan correctly. Good neighborliness will not fail because of us.

I have no idea about that. Can you explain why this becomes a problem?

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

That would really be the very first step. Immediately seek a conversation, then you can look further.

They build terraced houses and everyone wants to be the biggest individualist.

That is quite thoughtlessly written: I primarily try to protect our property. From the beginning, we informed the neighbors about the most important steps. If they do not then adjust to that, I do not want to be put in the pillory here.