Construction contract additional costs foundation reinforcement?

All relevant information is included in your initial post. According to static requirements blah blah additional costs. I also don't see any goodwill or meeting halfway. It's all just goodwill, nothing more. These are standard phrases in a general contractor contract that protects the general contractor. Exactly for this reason, fixed prices for general contractors are a myth.

He can hope for a compromise in that the costs for the reinforced slab should have been clear already BEFORE the contract was signed. It was possible for us as well. This allowed us to structure our financing quite differently (in our case, the soil replacement was also added).

In the course of the house calculation, the general contractor forgot to include possible additional costs, which could have been evident due to the existing soil report, in the fixed price. This is of course very annoying, but on the other hand, these costs would have been incurred anyway, so this is not a "general contractor problem" as it is currently being portrayed here. One can protect oneself against this by contractually agreeing with the general contractor that all additional foundation costs are included in the fixed price based on a soil report submitted in advance, provided that the soil conditions throughout the entire building do not deviate.

Soil report.
After completing the base slab, he informed me that the base slab needed to be thicker. Then came an email

It confuses me that the general contractor did not insist on a report.
By now, this is the guarantee that the house has.

And yes: everything is legal!
The fixed price of the house that you signed is based on the description of construction services, which contains something common like this:

in my construction contract it says

"Foundations and base slab"
Concrete and measurements according to static calculation. The base slab is a concrete slab with appropriate insulation that complies with technical specifications. The foundation is done with strip foundations....

However, it may be necessary to deviate from this. Usually, it is the plot of land, over which the general contractor has no control.
Naively and casually said: you should have bought a plot with better soil.
The contract for work is based on standardized processes. It can still happen to you that the temporarily stored soil cannot be used for backfilling. It would have to be removed. That is also not included for you.

We had the same: thicker base slab after soil ground report. Additional cost: €3800

If you want to make gold out of sh*t, you simply have to invest more. And your plot was delivered by you on the construction side.

Thank you for the many responses, we were able to agree on 50/50. Regards