Are additional material costs allowed in a construction contract despite a fixed price?

To my understanding, the price can be adjusted in a unit price contract if the actual quantities are 10% higher than those estimated in the [LV].

When the shell construction is finished, you can at least be relaxed during the negotiation. Usually, the craftsman has the upper hand there.

He has already built the shell and it is finished. So with ongoing construction, prices have changed, accordingly you can no longer withdraw from the contract, the problem now is that a surcharge is demanded which was not agreed upon before. The time frame was not that long, signed in March 2022 and shell finished in July.

The shell is finished, but how much of it has already been paid? If not fully paid yet, I would only pay the agreed price and possibly take additional earthwork efforts into account. But the steel price is basically a business risk in the first place. Then the contractor should properly justify why the agreed price no longer applies.

I had the conversation with him today and was proven right, the matter was resolved in my favor.