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

On what basis is she supposed to talk to the builder? She has absolutely no idea what she signed? Neither legally, nor professionally, nor technically, nor in any other way.

I agree, if one is at least somewhat familiar with the subject matter. But here it has been discussed for 25 pages and the OP does not give the impression that she understands any of what is being explained to her here. Then I see no other option than to bring in a specialist who can explain face to face how catastrophic the situation is... because my impression is that personal bankruptcy threatens if she can no longer get out of this contract.

On what basis is she supposed to talk to the builder? She has no idea what she signed at all! Neither legally, nor technically, nor professionally, nor otherwise.

I agree if one is at least somewhat familiar with the subject. But here it has been discussed for 25 pages and the OP does not seem to understand anything of what is being told to her here. I see no other option than to get an expert who will explain face to face how catastrophic the situation is... because my impression is that personal bankruptcy is looming if one cannot get out of this contract anymore.

Touché, but as is well known, it’s the tone that makes the music and so far that doesn’t seem to have fully registered with the OP. Understandably, since this is a really big crisis for an average person. That was mainly what I was concerned about.

But of course, both can be combined. You can seek an informal conversation, get input from the general contractor, and still go to a specialist lawyer afterwards.

One can look for a casual conversation, get the input from the GU

Without professional support, e.g., a construction expert, such a conversation from with the GU is pure waste of time, since she cannot assess the content at all. Therefore, I will quote myself

I would - make a complete inventory of all still expected costs with the help of a neutral expert - evaluate whether one can still afford this - get legal assistance to find out bindingly about the possibilities of contract termination and its consequences And after an overall assessment of these steps, presumably cancel the contract and then simply not build.

... etc.
Yes, what are we supposed to advise you? You write that you cannot spend more money. So what is left then?

I would
- make a complete inventory of all expected remaining costs with the help of a neutral expert
- assess whether this can still be afforded
- seek legal assistance to find out definitively about the possibilities of contract termination and their consequences
And after an overall evaluation of these steps, probably terminate the contract and then

You should - but I have already said this several times - clarify your situation with the specialist lawyer, get advice from her, and then proceed realistically. In my opinion, this will inevitably include reacting to the excessive costs by reducing the size of the house by about 30 sqm. I do not see burying your head in the sand and hoping for a miracle as a productive approach.

and what does reducing by about 30 sqm roughly mean in price? Yesterday evening I looked around for a building surveyor, I want to know what is coming our way

Generally, based on my experience, this has worked best for me privately as well. In every aspect. I am currently building my first house and luckily have no problems, but this is how I would approach it best there too.

I understand what your approach is and usually find it sensible. First seeking a conversation and remaining factual and polite is certainly not a wrong strategy. However, I do not see a contract partner at eye level on the other side here. To me, it seems like a company that is already hunting for easy targets. They won’t be easy to negotiate with because their business model is exactly what is happening right now.

In my opinion, this will inevitably involve responding to the excessive costs by reducing the size of the house by approximately 30 sqm.

I just have a brief comment. Simply shrinking by 30 sqm also entails (possibly expensive) consequences. I haven't read the entire thread, but is the floor plan such that you can cut out 30 sqm? Will the house still function (layout of rooms, pathways, stairs, size of rooms, etc.), or will a completely new house have to be planned (yes, I assume so)? New building permit or adjustment? And so on.