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

I would have to check in the documents.

Yes, have a look.
I would quickly have the contract including the construction service description checked by an expert for additional costs or positions that are included but for which claims are made that are not allowed.
Otherwise, there could be further surprises for you that you have not considered.

Yes, take a look. I would quickly have the contract including the construction service description checked by an expert for additional costs or positions that are included but where claims arise that are not allowed. Otherwise, there could be further surprises for you that you have not considered.

How do I find such an expert? What should I google for?

Construction expert… and then in your area

So far, I only remember having read the name "Haus der Handwerker" a few times from friendly but "muted enthusiastic" interested parties. Furthermore, you mention Massa Haus and Heinz von Heiden, from which you had alternative offers. So apparently you already had a rough house idea and were looking for implementers.

That you meant the right of withdrawal by "special right of termination" is good in the sense that, in my opinion, it now urgently should be exercised. However, I am a bit surprised to already read something about statics in your question – which means that some preliminary work must have been done for this to even be an issue.

You should seek independent planning advice, whereby architects regularly are the right address even for supposedly ordinary single-family house projects, and even more so for potentially difficult building ground.

I understand the concept of Haus der Handwerker as a mix of general contractor, planner and craftsman cooperative, and possibly also a bit of subcontractor job portal. I have several years of professional experience in owner consultation, alone with over 20k posts here, and I still have not really understood their concept – I don’t even want to imagine how bloody laymen fare in this regard.

Heinz von Heiden would at least be a classic (masonry) turnkey general contractor, albeit a big name, which I famously "do not prefer." Massa Haus, on the other hand, is a dedicated shell construction sales line of timber frame panel houses. So from a positive perspective, you are open in all directions, but one could also say: apparently thoroughly disoriented. At minimum, essentials such as whether you want to build "turnkey" or the opposite, a shell construction, should fundamentally be clarified before you set out.

Therefore, I recommend you first google "Bauglück" / lawyer Reibold-Rolinger. She offers a so-called "Builder’s license," which is a seminar for a few hundred euros specifically aimed at ensuring that laymen do not become easy prey for, let’s say, "moderately recommendable companies in the construction industry." With that, you should be fitter for your bride search and in basic legal questions for builders.

You specify your building location with "BW." Besides myself as a nationwide freelance construction consultant, I would also think of an engineering office in the Heidelberg area, which has already been discussed here and apparently practices the concept of a combined planning and consulting office: – and in provides a small overview of various providers (general contractors) in the region.

You mention your basement as 70 sqm as a "hobby basement," but not how far that was motivated by the need for ancillary rooms or by a possibly sloped terrain of the property. You can find explanations about when a basement is economically indicated via the forum search with the keyword "11ant Kellerregel."

Also, it "does no harm" if you present the construction project and its property in a separate thread. Please start this with Yvonne’s mostly ignored questionnaire:

https://www.hausbau-forum.de/threads/grundriss-planung-unbedingt-vor-beitrag-erstellung-lesen.11714/

how do I find such an expert? what should I google for?

Sorry to have to say this and I don’t mean it in a bad way at all. Maybe it’s also because of your stress right now. But if you have to ask such a thing yourself, then you can’t build with a general contractor you don’t trust. The information is just a whole Google search away. I am currently building with a general contractor with whom I have a very, very good feeling, and I still question everything and read into every topic at least enough to understand exactly what is happening and to be able to follow the site manager/the craftsmen exactly. Basic understanding for most trades should definitely be there. For the tricks and details, of course, you have the professionals. Read up on basements, what possibilities there are, and try to understand everything they tell you. Even if we all think together here now, you should always form your own opinion. You should have fully understood the construction performance description. Especially if you invest so much money in something. If you don’t want all the stress, then a large, well-known manufacturer who just does everything by the book is probably the safest.

If you don’t want all the stress, then a large, well-known manufacturer who does everything strictly by the book is probably the safest bet.

Whether the luck then fails because the customer wants to deviate from the book or because large providers with big legal departments are very skilled at legally taking advantage of laypeople is ultimately six of one, half a dozen of the other, right? If you want simple recipes, I still maintain that the construction company "Maurersohn und Enkel" from the neighboring town most likely keeps its name clean with impeccable work.