Building in existing structures - Subsequent additional costs despite a fixed price

The contract states "The fixed price is guaranteed, unless there are interruptions during construction of more than 2 months, which are not the responsibility of the general contractor."

My question was rather whether they can say, "if the additional agreement is not signed, we will not build and that is then your fault, so you have to pay more."
It is not even clear whether the foundation reinforcement is to be borne by me at all.

Of course they can say that. They can also say that they will not continue the work until you hop into their construction yard wearing the Adam's costume with an apple on your head singing O La Paloma.

What I want to say with this: Sit down with them (you already have a management meeting). The forum will not help you here. Either resolve it amicably or through a lawyer, but then mentally distance yourself from the house.

By the way, the definition above is extremely negative for you. If they can't get any part for over two months, they are allowed to raise the price without you being responsible for it. It should say "due to delays caused by the client" or something similar.

The responsible site manager has now given us an ultimatum

I’m saying, being right and getting your rights are two different things. And being right and getting a house are also different. That’s why the very first question was whether it would be possible to terminate the contract. Legally, the ultimatum is certainly not clean. If the general contractor’s claim were legitimate, it would still remain even if you don’t sign anything. I wouldn’t react at all and would have the lawyer send the corresponding letter to the general contractor by fax on Friday. Content probably: demand to continue construction

Sign the paper, pay the bill and tell the CEO on Tuesday that this was your absolute concession and that you can now certainly expect a swift and great construction

Unless you don't want to move in until 2025, then assert your rights and make a new contract for 20% more or so...

Understand yourself, but here you can only lose unless you build the house without emotion and can wait...

Sign the paper, pay the bill, and tell the managing director on Tuesday that this was your absolute concession and that you can now certainly expect a swift and great construction

Unless you don't want to move in until 2025, then enforce your right and make a new contract for 20% more or so... I understand you, but here you can only lose unless you build the house without emotion and can afford to wait...

But that doesn’t solve the problem. The contractor is already threatening not to adhere to the fixed price. (In the end, there will certainly be more than 20%) That is probably his goal. Without establishing facts here, the original poster is at a disadvantage. Delays in the construction process are the contractor’s responsibility. He should first prove that he does not owe the additional foundations. The specialist lawyer will answer that question. As a precaution, I would announce damages for delays caused by the contractor. These should also be properly documented with the negotiation protocol of the meeting (a side effect is the documentation of the attempted extortion), which should be sent to the contractor.

There could be a clear description of the contract content, where the service to be provided is described in detail. If foundations are not included, the legal situation can certainly be questionable. The lawyer helps here (he knows the contract). Fixed price only means that the services included in the contract are fixed. It looks different if a lump sum offer became part of the contract content.