Price increase despite fixed price!

The law states:
(1) If circumstances that formed the basis of the contract have changed significantly after the contract was concluded and the parties would not have entered into the contract or would have done so with different terms if they had foreseen this change, adjustment of the contract can be demanded, insofar as one party, taking into account all circumstances of the individual case, in particular the contractual or statutory allocation of risk, cannot reasonably be expected to adhere to the unchanged contract.

For example, I see that a proactive entrepreneur in January had to expect further price increases. At least my general contractor commented in January that he was already receiving numerous announcements of price increases of around 10%. Since only the material has become more expensive, but wages certainly have not risen in an unforeseeable way in the two months, I see little that corresponds to this.
Values like "20%" may come from practical experience as guidance, but the law does not mention this.

Not to be taken as taking sides for the company here, I count Weton among the recommendable companies (and as far as I know was satisfied building with them), so that I ultimately do not expect "an improvement" ;-) if the original poster now looks for another construction company (or even takes the kamikaze course of trying a single private contract).

Values like "20%" may come from practical orientation, but nothing about that is mentioned in the law,

No, it is not stated in the law. That is an amount that comes from the OLG case law. The BGH, on the other hand, says that these are individual case decisions. The hurdles for § 313 are very high, the general risk of price increases always remains with the contractor. Quote: "The contractor is entitled to a price adjustment only under the (high) conditions of the disappearance of the basis of the transaction according to § 313 of the Construction Code. It is at least necessary that the circumstances outside the control and risk area of the contractor after the conclusion of the contract have changed to such an extent that a gross disproportion arises between performance and consideration, making it no longer reasonable to adhere unchanged to the contract (Palandt § 313 margin no. 31f; BGH BB 56, 254). It must be taken into account that, as a rule, the contractor bears the risk of price increases for his service (OLG Düsseldorf judgment of 19.12.2008 – 23 U 48/08, BeckRS 2009, 331; OLG Hamburg BauR p. 680, 681)."

What a joke!!!!! Seek a conversation, reject, if necessary with a lawyer.
Put differently: Who pays for the fun of prepayment penalties when withdrawing from a loan? Who pays you the opportunity costs of the now much higher interest rates?
That would be ridiculous if it were that simple...

The homebuilder should just give up 10% margin and only earn 5% or 10%...

And the total prices have certainly not increased by 20% overall because of the war (the wage component alone makes a significant difference in house construction)

I can well imagine, however, that a few would agree without grumbling, that would already be a win situation for the home builder... But who just has 50k or 100k lying around at those prices to chip in?

if necessary with a lawyer.

I just had a Freudian slip and read "if necessary with VIolence"...