Subsequent structural change/costs

Hello,
we have concluded a consumer construction contract with a house building company. The contract contains the clause that due to ordered measures with the building permit or in the calculation of the statics, (subsequent) additional services may arise.

We exchanged the wooden staircase against a concrete staircase compared to the type house we selected. The seller offered us this change at that time for an additional price of about €200 and it was included in the contract before signing. After the contract was signed, the structural engineer calculated at some point, the building application and the building permit were completed, the house building company now wants to claim additional services due to the statics. The ceiling between the ground floor and the upper floor is to be changed from 18cm to 22cm because of the staircase. This is supposed to cost about €1000. So five times the original €200. Is this lawful? Or where is the limit?

Thank you very much for the answer(s).

I would say the 200 euros were only the additional cost to replace the stairs. We also paid the extra 1000 euros for the structural engineer because we needed a stronger base plate due to the pile foundation.

I would say the 200 euros were only the additional cost to replace the stairs. We also paid the extra 1000 euros for the structural engineer because we needed a stronger foundation slab due to the pile foundation.

Yes, of course 200€ was the replacement. But it can't be that the whole process actually costs 1200€ afterwards. That amount should be included on the price tag from the beginning. Or not?

Yes, sure, the exchange was 200€. But it can't be that the whole process actually costs 1200€ afterwards. You have to put the price on the tag from the start. Or not?

Nope. The structural engineer is mentioned. Additional cost

But that can happen. Depending on what is in your [Bauleistungsbeschreibung]. Often it says something like "changes to the floor plan can lead to additional costs in the structural engineering" or something of that kind. Of course, you should have been informed that a heavy staircase leads to a changed structural engineering, but really very few sellers do that...

Nope. A structural engineer is mentioned. Additional cost.

Yes, of course, but there has to be a financial cap. It can't be that I buy a house for amount X and then the structural engineer comes and says what still needs to be done and suddenly the price becomes 5 times amount X.

In my opinion, there is a 10% rule for craftsmen's bills. An invoice may deviate from the quote by 10%. There must be such rules in our case as well. There has to be a limit.