Termination of basement construction contract - experiences?

Embarrassing... but I can't find it in the contract

Then it is therefore the Building Code… Otherwise, you would know it.

If I remember correctly, you are building with EBK (i.e., a Danish manufacturer with a German ichsachma importer or factory representative). A German basement builder might (possibly for good reasons) have doubts about that, and presumably a language barrier (technical communication exchange in Danish/German) does its part as well. Unfortunately, I share that concern in the end. The basement execution planning should be done by the basement builder; he knows his system best. From the house manufacturer, he needs information about the positions and dimensions of the openings in the basement ceiling, and about the load transfers of the house.

1. On which specific problems does the basement builder base his impression?
2. What do other customers of the importer/factory representative say about how the "substructure" of the house was handled for them?

In this constellation, I see "reproducible" problems that could continue with another basement builder (and I would check whether I might be parting ways with the wrong troublemaker here).

I suspect so too, because the Construction Code applies if nothing else has been contractually agreed. So if you really want to separate, the 8% would probably reflect the "justifiable reimbursement claims" by the contractor, which according to the Construction Code must be paid upon contract termination. I recall that in a contract under the Construction Code in construction law, it is regulated that the services and expenses provided by the contractor up to that point must be remunerated, and the services listed in the contract but not yet rendered must be credited back. But here’s the catch: If the contractor cannot profitably use or offset the planned capacities through other assignments, further claims could arise against the client. The burden of proof then lies with the client, and that may be difficult if the contractor is not cooperative – which I would not assume. Either you manage to establish a climate of necessary cooperation, or you separate with the 8%; honestly, I don’t see anything in between and it would probably end in chaos. *Edit* I would try “everyone at one table”. Organize refreshments for everyone (you obviously don’t sit down to eat with enemies) and have an open communication about the little problems and complaints. Only if that doesn’t work would I consider an exit.

Pulled through? No.
Experience with contracts? Yes, and therefore also a very simple answer. Book the 8% under pP (personal bad luck). Pacta sunt servanda = contracts must be adhered to. You signed an 8% penalty, so you have to pay it.

So indirectly you are also to blame that the basement construction cannot work because it does not receive an execution plan? No offense meant, but as an additional argument from the basement construction side, I would say this besides the fact that you have a contract.

The only little door you can still go through: The contract with the basement builder is contestable for whatever reasons. But that costs money because you have to pursue the "fun" legally and maybe for a long time. If you start with another basement builder in parallel without having terminated the old contract, you could of course still be liable for damages to the old basement builder, provided you do not get a right.
But that is just my layman legal opinion. For more, the "law" part of my studies is too long ago.

P.S.: I say again for good reasons that you should never order a basement yourself, but include this delivery and service package in the contractual scope of the house provider. OKKD is a critical interface!