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

Would you like to post the relevant passages here? Especially the one about the basement and the additional costs?

I still find it dubious what they mean by that. As far as I understood you, they told you that a bitumen waterproofing was included, then you had to pay extra for a black tank, and now even more extra for a white tank?

That seems strange to me, because I always thought the bitumen waterproofing IS the black tank. And a white tank is per se not an upgrade: These are two different methods, each with their own pros and cons. One disadvantage of the white tank, as far as I know, is that it is more difficult to construct and must be executed very precisely, otherwise your basement might not be waterproof.

Feel free to contradict me if I’m wrong. I’m not 100% into the subject, but it already sounds dubious.

If you have to go through with the contract, it’s all the more important that you get someone on board who knows about construction and supports you. Trying to push through this thing alone, with the money that’s available, I consider dangerous. In the worst case, you’ve been squeezed out of your money (and it’s a lot), and you end up with a half-finished house or a construction ruin.

Someone who knows about construction law wouldn’t be bad either, but that doesn’t necessarily make it financially easier. Regarding the contract, the kid is already in the well.

We paid 9k net for a “real” architect up to the execution planning stage, just for reference!

With a prefabricated house, the "architect" of course has much less to do than with individual planning. It's piecework.

I have to say that h&h had noted in the documents that it might be necessary to have a bathtub and that this would involve considerable additional costs. But that we are in earthquake zone 3, they could have told us earlier or we should have informed ourselves better.

The provider has no obligation to inform you in advance about the necessary extras for your plot. They only have to clarify what is included in the contract. You are overwhelmed with this matter and urgently need someone who understands construction contracts and can tell you what additional costs you will face and whether you have a chance to cancel the contract, to amend it in a reasonable way (if trades are removed, the planned profit usually remains with the company), to change the planned construction, or to arrange additional financing.

now the construction company wants us to also waive the special termination right in writing - is that legal?

Well, besides me, quite a few have advised you against that.

Now you write this:

we actually had to waive our special termination right as well.

Why?? Did you really waive it?

the h&h stated in the documents that it is possible that a bathtub is needed and then significant additional costs will arise.

Then you surely set aside 50k as risk coverage. What did the neighborhood do? Built without a basement?

Then you have certainly set aside 50k as risk coverage. What did the neighbors do? Built without a basement?

The neighbors have partly built without basements and partly with concrete basements.

Several others besides me have now advised you against it. Now you write this: Why?? Did you really give it up now?

they said they cannot start work planning otherwise.