Building in existing structures - Subsequent additional costs despite a fixed price

If I understand you correctly, the contractor expected the basement to be larger than stated in the documentation (which is also mentioned in the contract). Have you asked him how he came to that conclusion? I would assume, if there are no indications to the contrary, that the basement is exactly as large as stated in the documents. That's where the problem lies, isn't it?

Was the plan for the new building already finalized at the time of signing the contract? Has anything changed in the plan after the contract was signed?

It is not about the entire basement, but exclusively about the middle foundation strip. This is executed as documented, but it was later determined that it does not withstand the new load.


The plan for the new building was settled at the time of contract signing. Nothing has changed about it.

What would you consider as a "mutually agreed solution proposal" taking into account the circumstances I described?

Addendum: In the construction description, the topic of the exterior wall is of course specified in more detail. It states 24cm Poroton + 16cm mineral wool.

Regarding the important point about the basement, however, only the exact wording I wrote is given.

As an expert in formulations, my question to you: This is hardly legally secure wording for any party. Why do you assume that it would be formulated to the advantage of the GÜ and not disadvantageous? There are no reservations or other references and restrictions in the contract.

It even explicitly refers to the historical execution documents, or does that carry no weight?

@11ant What do you mean by "the contractor has to [...] inflate the order volume."?

Well, the construction boom is declining, and he has people he wants to keep employed. So he tries to increase the volumes of existing orders, like adding an apple turnover to a burger.