First alleged fixed price and now additional costs - legal?

And what is supposed to be included in the 200k? At that price, I rather suspect that another 100k and not just 10k will be added on top....

Has the soil survey been included as a basis in your contract, or does the standard phrase covering everything up to 30 cm apply? In the latter case, additional effort would have to be compensated.

The standard phrase with 30 cm is included. The seller inspected the site, surveyed the slight slope, and said that this is included in the price. He has the soil survey from me. However, the survey specifies a greater thickness of the topsoil layer. At the planned construction site, I am also concerned that since the new house is supposed to be built where the old one used to stand, this opens the door to further additional costs.

And what is supposed to be included in the 200k? At that price, I suspect that another 100k and not just 10k will be added on top....

Foundation, house, without painting and flooring, thin walls, everything kept simple.

I haven't paid anything yet. They are still waiting for a signature from me before they continue with their own soil expert. It is still unclear where the house will be. However, since there used to be a house on the building plot, I am concerned that they will request further additional charges for the building ground.

The contractor's expert will assess what is certainly not in yours, so additional costs are very clearly to be expected. One expert opinion is completely sufficient. In the end, you will also pay for the contractor's expert opinion. Reject these additional costs immediately before the expert opinion is commissioned. If the building ground does not correspond to your expert opinion, you can agree with the general contractor before ordering a second expert. Then your expert belongs on the construction site. Additional costs because of the house are quite easy to reject; it is stated in the expert opinion that there was a house there. Whoever overlooks that is their own fault.

Foundation, house, without painting and flooring, thin walls, everything kept simple.

And the usual remaining €50,000 ancillary building costs, have you planned for those? Do you have the section about the failed financing in the contract?

Seller inspects on site, measures the slight slope and says that this is included in the price.

Do you have what he said in writing? Sellers often say a lot and then have big memory lapses.