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

You should have deleted the attachment right away. It only lasts 3 minutes. You can try to ask the mods. I see little chance there.

A good site manager can also evaluate the supplementary claim for the basement white tank. The one from the general contractor won't be any help there. Rather the opposite. He profits from the contract with the basement builder.

"A good site manager can also evaluate the supplementary claim for the basement white tank. The one from the general contractor won't be any help there. Rather the opposite. He profits from the contract with the basement builder."

The site manager said that in an earthquake zone level 3, you are not allowed to build a basement without a concrete basement.... is that true?

Your site manager does, but not the one of your contractor. His tasks are: 1. that no construction worker stands empty-handed due to late-delivered materials; 2. moderation between the subcontractors; 3. effective concealment of defects until the warranty period expires. He has nothing to do with your interests, contractually you are the counterparty as the service creditor!

But you have forgotten quite a few important things.

The specialist lawyer can still get the child out of the well, and the debt counselor can then dry it as best as possible. No, not a word more without the lawyer – and every hour earlier she is involved counts!

But let's assume that the general contractor would be accommodating, be it out of goodwill or whatever. That of course dies immediately the second a lawyer calls there. This doesn't quite fit now, but I myself manage a department in an IT company. Occasionally, I like to be accommodating to customers out of goodwill, even if it costs a few euros. Especially if my employees/myself are treated honestly and fairly. However, as soon as I receive mail from a lawyer, I am naturally forced to contact our lawyer immediately, and from that point on, accommodation for both sides is dead. That can then potentially strain the entire business relationship, and that's exactly what you really don't want during a house construction. I am a fan of always trying to have a conversation first. Nothing has to be decided there, but at least you can exchange views and clarify questions. Often the interpersonal aspect makes a difference too.

The construction manager said that in an earthquake region level 3 you are not allowed to build a basement without a concrete cellar at all..... is that true?

Sort of. A structural engineer can definitely answer that for you. A forum rather not. I would claim that in earthquake zone 3 only a concrete cellar may be built, as it can be particularly reinforced. Whether it necessarily has to be a waterproof concrete shell (Weisse Wanne) right away? I could imagine that it might also work with concrete-filled bricks. Whether that will ultimately be cheaper? Then there is also the sealing to consider (black tank?).

That can then possibly also burden the entire business relationship, and especially that is really something you don’t want when building a house.

The OP is already too timid, who lets a tiger treat her like a lion. She is already nervously accepting the budget for the kitchen and contingencies being redirected into at least 60% additional costs as supposedly without alternative. Powerlessly, the general contractor throws her against the wall without even trying.

But let's assume that the general contractor would be accommodating, whether out of goodwill or whatever. That, of course, dies immediately the second a lawyer calls there. This doesn't quite fit now, but I myself manage a department of an IT company. From time to time I like to accommodate customers out of goodwill, even if it costs a few euros. Especially if my employees/myself are treated honestly and fairly.
However, as soon as I receive mail from a lawyer, I am naturally forced to contact our lawyer directly as well, and from that point on, accommodation is dead for both sides.

This attitude is regrettable, unwise even for your work, and not a good influence on the OP. Legal advice does not mean lawyer’s call or lawyer’s letterhead. And lawyer does not mean confrontation at all. I consider the mentioned specialist lawyer capable of successful mediation, and the OP should gladly also respond discreetly – for example, sending the general contractor a letter written by the expert attorney in her own name. Don’t scare the OP into fearing escalation!

I could imagine it works with concrete-filled blocks as well. Will that end up being cheaper?

Do you mean formwork blocks?
I’m assuming here a downgrade* to slab foundation, reduction of house area, and above-ground basement replacement space done by herself, otherwise the OP would be paying forever with asset damage for the naïve signature.

*) mind you after the rescue by specialist lawyer and debt counselor!

It says "Basement according to plan". What does it say there about the basement?

It is almost irrelevant when you consider how many check marks in the offer suggest that it is included... but it is only part of the offer