Building with a general contractor - claim for additional costs shortly before the start of construction

The TE has already accepted the offer with the corresponding commissioning of the planning. Thus, the offer prices are binding. The creation of a deviating contract is to be rejected. Reference is made to the fulfillment of the already commissioned offer. But I am not a lawyer. What exactly is stated in the offer? Was there a price binding?

So now I have to ask a stupid question. If I sign a construction contract with a sum X and there is, for example, no sentence like "The price is valid for 6 months after acceptance" or something like that, then I assume it is a fixed price. Or am I seeing something wrong? Otherwise, anyone could just come and demand more.

If I sign a construction contract with amount X and there is no clause like "The price is valid for 6 months after acceptance" or something similar, then I assume it is a fixed price.
Or am I seeing that incorrectly?

Yes, you misunderstand the term "fixed price." It is a price that is not tied to a specific mass/quantity (which can vary between order and measurement), but is considered agreed upon regardless. A guarantee against passing on interim price increases is neither associated with nor included in it.

It's not that simple. It depends on what was agreed upon. Is it an offer based on the VOB or according to the Building Code? Generally, it is not common to agree on price escalation for the construction of a single-family house, since the construction time is usually very short.

At least there seems to be a change in thinking, as nowadays there is no longer a short construction period and certainly not predictable price increases from suppliers. However, this requires a separate regulation already in the offer. I do not see that here based on the description of the OP.

Yes, you misunderstand the term "fixed price." It is a price that is not tied to a specific mass / quantity (which can vary between order and measurement), but is considered agreed upon regardless. There is neither an associated guarantee nor included promise of not passing on interim price increases.

Where did you get that from? That is nonsense.

Where did you get that from? That is nonsense.

Why? There are differences between a "fixed price", where quantity changes etc. are not taken into account, and a "flat fixed price", which is truly unchangeable.