Right of retention and refusal to accept, whose fault?

That's correct! The architect must be licensed in Germany.

I have to honestly say that I have a big problem with the complaint about defects in terms of content, substance, and form.
A complaint about defects is short, clear, and concise and is not an instruction to the addressee.

I sent the message below to the floor slab company.

Ultimately, not only that, the request for a written receipt of the message is missing. I don't know if email is sufficient. I would always send it by registered mail with return receipt via post.

Your specified date was 22.12.
I assume he hasn’t responded? Or has he gotten in touch?

I cannot say anything technical about it; I would have such matters handled by the architect. But in this case, it is also somewhat "unusual."

Let me make it clear: You messed up yourself, the construction company will not pay for it. Even your [rechtlicher Beistand] won't change that.

You would have been better off investing the money for the [rechtlicher Beistand] in proper planning / construction supervision...

But now the horse has bolted, so look for solutions instead of blaming others.

I'm interested in that too. We had to submit the certificate of our architect's qualification and numbers (professional chambers? register?) "everywhere" to the authorities for the building permit, etc. For the utility providers, we had to provide proof of the qualifications of various installers and also numbers (in chambers of crafts? register?).

Slovenia (is part of the EU) and yes, of course we entered the numbers.

That’s right! The architect must be licensed in Germany. To be honest, I have a major problem with the defect notice in terms of content, form, and substance. A defect notice is short, clear, and concise, and not a lecture to the recipient. Ultimately, not only that, the request for a written confirmation of receipt of the message is missing. I don’t know if email is sufficient. I would always send it by registered mail with return receipt via post. Your stated deadline was 22.12. I assume he did not respond? Or did he get in touch? I cannot comment on this professionally; I would leave such matters to the architect. But in this case, it is somewhat “unusual.”

I didn’t copy everything here and yes, I sent it by mail. He got in touch and refuses to acknowledge it.

We here in the forum only have the information you give us. So far, it is very incomplete. To me, the story reads like this: you are a total layman, have a cheap architect from Slovenia, and no separate building expert. From experience, such listed defects usually end in a financial disaster. Realizing that you are largely to blame yourself, then quickly counteract and hire an expert is, in my opinion, the only way to still get away with a relatively minor loss.

First of all, to this day you have not plausibly explained how the additional costs of about 450% for the removal of the excavated soil and the construction of the driveway almost, but fortunately did not come about. If I had fallen into your trap and paid the final invoice, I would be twice as badly off today. Many defects have come to light, but I will first list the worst here: [...] I now demand that you eliminate the above-mentioned defects by December 22, commission someone else for this, or waive the final invoice as compensation for the defective work.

Unfortunately, we can understand very little of what you are telling here very fragmentarily. More badly than well, I could piece together that you probably want to build a prefabricated house from a Slovenian manufacturer with a correspondingly Slovenian plan author in Germany, where the site work and up to the upper edge of the floor slab are to be carried out "by the builder" and were subcontracted by you yourself here. Who and in what "quality" feels responsible for usable working plans of the floor slab in this constellation probably has the devil as referee. A duty of a German floor slab pourer to re-assure himself with the suppliers about the compatibility of the indicated components with their regulations is, to put it mildly, "unknown" to me. Your notice of defects has the potential to become legendary in the joke section of a law seminar. If I were a lawyer, I certainly would not take your mandate after such an own goal. Alone the five and a half times cost increase of an obvious lump sum speaks volumes about the sloppiness of the tender— I can hardly imagine that the content description of the order would be even remotely suitable to examine liability attributions. Proper defect notifications, by the way, are complete enumerations (and) without any tearjerking.

How can a foreign architect work in Germany? Not at all, so someone else is liable.

I really cannot imagine a transnational plan submission at a German building authority either. To my recollection, the American lawyer Goldfine needed a German partner firm at the time in order to represent the construction lion Schneider in Frankfurt in court. According to my knowledge of the German chamber system, I also do not believe in international openness in this case.