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

sure, there are other good construction companies too but I wouldn’t have paid less than at h&h there. and with contaminated soil I would have problems with any other construction company as well

and the soil contamination test costs 1000 euros

You might have paid significantly less with another construction company, possibly. At the end of the construction, when everything is really finished. You are comparing apples and oranges here. That would be like comparing a car you buy at a reputable dealership with a car where someone offers you: "This car costs €10,000 less than the one from the other dealership - complete! (* Headlights cost extra. Windshield wiper system costs extra. Tires cost extra. Window regulators cost extra. Gear oil costs extra. Fuel tank costs extra... we have a large selection of alloy wheels (which cost extra))."

This can't be the complete contract... my construction contract has 52 pages...

This is just an offer

just as a side note - with Hein von Haiden the city villa with basement would have cost me 530,000 euros!! over 100,000 euros more and I wouldn't have been allowed to move the walls but only exactly as in the catalog

If I put together the last 20 pages correctly, then you are defending a company here that wants to sell you a basement without waterproofing and at the same time wants to charge you 30k€ for the waterproofing (black tank). Now at least it will be a white tank. By the way, that is not a protected, strictly defined term either. Here the quality / the mixture of the concrete is relevant. However, the construction company is not the problem here. You simply have no clue right now, so the general contractor does whatever he wants with you & he is doing exactly that.

This is just an offer
(...)

This is an advertising brochure.
An offer includes an exact description of what will be done and, above all, what it costs.

However, the construction company is not the problem here. You simply have no clue whatsoever, so the general contractor does whatever he wants with you & that is exactly what he does.

I see both parties as the problem. On the client side, an explosive mix of ignorance and resistance to advice. And on the construction company's side, dubious business practices where houses are sold at rock-bottom prices to naive customers, only for half of the costs to be added later as additional charges. I am building for the second time now, and neither of the two construction companies pulled or even attempted anything like this with us.

This is just an offer If I piece together the last 20 pages correctly, you are defending a company that wants to sell you a basement without waterproofing and at the same time wants to charge you 30k€ for the waterproofing (black tank). Now it will at least be a white tank. By the way, this is not a protected, strictly defined term either. The quality/the mixture of the concrete is relevant here. However, the construction company is not the problem here. You just have no clue about anything right now, so the general contractor does whatever he wants with you & he actually does that.

Well, it was our mistake that we did not get better informed regarding the basement. It did say in the offer (cover sheet) that depending on the soil type, considerable additional costs would arise. So it would have been our task not to have the soil survey only as part of the work planning but to do it independently before the contract.