How much additional cost according to the structural engineer's calculation?

You really planned to just dig a basement into the ground without any waterproofing or anything else, and just wanted to lay drainage around it? If that was really the plan, you urgently need someone to advise you. I can't imagine the builder wasn't aware beforehand that this wouldn't work.

yeah, waterproofing of course but now we need the black tank, which will surely be an additional cost of 15,000 euros

I can't imagine that the BU wasn't aware before that this was not possible.

That was clear to him. But why should he point that out to the uninformed builder beforehand? That's exactly how you make a profit when building. If I were to review such a change order, the first thing I would ask is what has changed in the geology compared to the original calculation. Is the groundwater level higher now? Is this a fixed-price offer? How exactly are the basement and drainage described?

The price would have included the normal sealing and drainage, and now we need a black or white tank.

What exactly is stated in the offer / contract now?

yeah sure some kind of sealing but now we need the black tank, which surely means an additional cost of 15,000 euros

I thought the black bitumen coating is called black tank. Hmm…

Since you seem to know your stuff well I wanted to ask what the soil removal roughly costs, nobody could tell me that either.

As well as you see, not really. For us, without a basement, quite a lot of soil accumulated. Over 100 cubic meters of loose soil. We had to hurry to get it away, since the neighbors wanted to start as well.
We found a buyer via eBay classifieds: but the property was about 20 km away, so with one day of commuting by truck, not much was moved.
My husband then simply asked a construction company at a site, who disposed of the soil for 2500€. That was 10 years ago and basement excavation nowadays surely costs 8-10,000€. Maybe a reader here will chime in with more precise info. Unfortunately, I can’t think of anyone active here who has built with a basement. (Except on a slope, like ). Exactly because of the horrendous costs, basements are dropped by many nowadays.

Is this a fixed price offer? How exactly are the basement and the drainage described?

I suppose the basement is a separate item in the house offer, ?
How much does the basement cost in the offer?

I would be interested as well. What kind of sealing was planned if it wasn't a black tank? Isn't that the "minimum" you can do in new construction?

I can't consider the construction company to be reputable with such business practices. Usually, there's a reason why they appear cheaper than Heinz von Heiden or Massa Haus. Bait prices where then many things are left out that a layperson doesn't notice.

First, the dubious sealing is "upgraded" to the technical minimum. Then the house suddenly becomes more expensive due to the static calculation. Why? What has changed? If you want to stick to the contract, I would really make sure to get someone on board who knows what they're doing.