Surcharge due to rising prices

That is more than fair from the contractor, considering that prices for some trades can quickly double. If I were you, I would reverse that immediately, otherwise your general contractor might stop working with you altogether, and you will have to find someone else for 30% more. And that can quickly mean over €50,000 for you. Besides, you gain nothing if your general contractor becomes insolvent during construction. Then it gets really expensive and annoying and takes forever.

Even though I basically agree with you that the original poster is getting off with less than a scratch at 5.5k... you can't just change a fixed price and if the construction company then stops responding after announcing the price increase, that's not good either.

Thank you for the helpful answers. The surcharge would mean 5500 euros for us. The matter has been with the lawyer since today. I will gladly let you know how it ultimately turned out – for everyone who is affected themselves. I can imagine that this is not and will not be an isolated case.

That you are having the matter legally assessed is, from my point of view, fine. Knowing whether you are "in the right" or not can be emotionally helpful and relieving (after that you can still decide whether to act "generously" or "assertively"). If, apart from the 5,500 euros, there are no further difficulties between you and your preferred home builder, it would be purposeful, regardless of the lawyer's advice, to analyze your situation and, if you still want to build with this home builder, to accept the price increase without legal intermediaries.

Having the matter legally assessed by you is, in my opinion, acceptable. Knowing whether one is "in the right" or not can be emotionally helpful and relieving

... but remains irrelevant for commercial reason. Anyone who wants to gauge the luck potential of being right only needs to enter "Fleischerhaus" into the forum search ;-)

Even though I basically agree with you that the OP will come away with less than a black eye at 5.5k... a fixed price cannot simply be changed, and if the construction company then stops responding after the announcement of the price increase, that is also not good.

The fact is legally, in the described situation the contractor has apparently unlawfully shortened the price binding period. On the other hand, he would have a whole toolbox of unpleasant (though not legally actionable) ways to "take revenge" on the client if he refused the solidarity here which is probably comparably "symbolic." With the contractor presumed by the OP’s nickname, organizational structures are likely to be assumed where the described communication quality is seen as customary according to our experience. Whoever does not like this culture must not go to the big names but must build with the small bricklayer master around the corner.

I would still be cautious. You are giving up the fixed price of 5.5k. What is stopping the contractor from demanding another 50k?

Or have I overlooked something?

Here, one cannot judge anything precisely at all, as neither the text of the original contract nor the complete "Aufpreis" scenario is available. This is pure speculation.