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

No, it is the black tank.

Not yet.

Soon it will be the additional reinforcing steel, the changed construction time, the bad weather, the surplus soil excavation, the soil replacement (since no load-bearing ground is present during excavation), material price increases, wage cost explosion due to extreme collective agreement, and so on and so forth.

I understood the OP to mean that they once had a buffer that would cover the black tub, and the site manager has now said that an additional mysterious surcharge will come after the statics.

It's already stated in the headline.

Slso, I honestly don't understand the concept of the company HDH.

I do. It's called change order management.

That's lesson 1 in the site manager seminar.

You may be right about that. For how many square meters? Proper storage basement or for residential use?

The basement is 70 sqm. It is not for living, only a hobby room. However, the basement will not be finished with the price but will remain in a raw state. Only the anteroom will be finished.

I understood the OP to mean that they once had a buffer that was intended for the black tank, and the site manager has now said that after the static calculation, there will be another mysterious surcharge.

That's right. However, we haven't factored in much buffer...

How is it sealed according to the contract and is it made of concrete? Is a plan part of the contract?

I would have to check the documents.