Surcharges of 10% despite a fixed price contract

Out of curiosity. How many sqm, basement, equipment and what price?

Otherwise, it probably means having it legally reviewed there. I would try to talk to the GCs and see where the problem lies.

We also still have a Bafa subsidy that will expire for us in February 2023 if our project is now delayed.

Although I am not a lawyer, I would estimate that your contracting partner also risks liability for damages.

The entrepreneur has no choice but to pass on price increases as much as possible or ultimately face insolvency.

I assume that an attempt is being made here to test which customers will put up with this game. The calculation might be that a three percent increase is expected, every third customer will accept it, and then ten percent must be taken from those for it to work out (which, for the customer, would initially mean: those who complain will not be dealt with at all). However, both parties (the general contractor and the customer) overlook one danger: if many customers become nervous and doubt the general contractor’s financial standing, inquiries to credit rating agencies will skyrocket, and the general contractor can get into insolvency purely through the self-perpetuating dynamics of credit doubts, even without any deterioration of the hard facts. Instead of the original poster, I would therefore also discuss with the lawyer how securely the contract can be terminated without damage, and in any case remain especially vigilant regarding the "securing of contract fulfillment."

The 10% are annoying and not insignificant. Apart from the fact that a lawyer's letter provides an optimal basis for a trustworthy collaboration, the price increases are a problem. What is happening at the moment could not have been foreseen by anyone. The entrepreneur has no choice but to pass on price increases as much as possible or ultimately face insolvency.

Well, if that were really the case... no one could have foreseen it... Corona has long been factored in. The general contractor certainly included possible price increases in his offer and will not be hit so hard that it goes straight into insolvency. And if he did not do that, then that is his problem. Or does this general contractor ask the client how he is coping with a 10% price increase?!

…a funny "counterargument" would be that due to the current situation a 10% increase is not possible and instead a 10% reduction of the total price is necessary. Fuel prices and so on… Seriously: contract is contract. Payments strictly according to construction progress from now on and really check carefully.

You could also just come and say the interest rates are 1% higher than planned and therefore the financing will be 30,000€ more expensive....

As you describe it, this is blackmail. Send a letter and demand contract fulfillment. Since there are doubts that he can fulfill the contract, require a performance bond of at least 5%. Claim damages in the event of non-timely contract fulfillment. Your lawyer will tell you exactly what to write in that regard.

Why didn't your lawyer advise you to demand a performance bond?