Process fixed price guarantee - Adjustment according to construction price index

Hello forum community,

we received today, as expected, an additional charge of €33,000 because we have now exceeded our 8-month fixed price guarantee (in hindsight, we know that the start of construction within this time frame would not have been possible from the outset).

Now the house is almost finished as far as the shell is concerned, the first 3 installment payments have already been made by us. The additional charge has been added to the total price and is to be paid from the 4th installment (finished shell with roof truss and roof) in proportionate stages.
The additional charge is calculated based on the construction price index for residential buildings, which is also stated in the general contractor contract. There was only a printout of the construction price index and that was all. No detailed explanation of what exactly has become more expensive.

We would like to know if there are people here who have similar problems or if there are strategies that have at least made a partial waiver possible. Basically, we know that there are corresponding price increases. If so, the price increase should be justified in detail and not done on a flat-rate basis. It is also questionable whether we have to share the relevant profits of the general contractor.
Experience regarding this would be very helpful for us, as there are currently additional charges in amounts that were not factored in for us when the contract was concluded in September 2021.

I would cautiously put my finger on this: 3 sections already paid for, so the index increase is insignificant for them. You might be able to get more out of it, but then it will just take longer because unfortunately the construction company has the leverage here, not you. I would only negotiate or have it clearly stipulated that this is really it now and that there won't be another adjustment in 3 months... currently, you are continuing to build without price protection.

The reason is irrelevant and you also have to bear the profit. At least according to your post, that's what the contract says since everything is based on the index. If you had wanted it differently, you would have had to negotiate.

We would like to know if there are people here who have similar problems

There are, for example here: Legally speaking, you are in a weak position, construction cost index is construction cost index. In the end, only negotiating helps. But the interesting question is what exactly the conditions for the delay were. 8 months is rather tight for a construction, before that the permit must also be obtained and the construction site scheduled. There may be leverage here, e.g. preparation of the building application by the construction company took 4 months.