GU does not guarantee KfW compliance. Cost risk to the customer?

If I order a new BMW, the leather seats cost a 5,000 euro surcharge and BMW then says "we don’t yet know if the cow will be in the mood that day or if there are enough cows, it may also be that in the end there is no leather",

... not everything that is a faulty comparison actually compares: unlike milk, the cow only gives its leather when it is dead.

you write an offer, important is what is stated in the contract. If points in the offer already bother you, have the contract draft reviewed by a construction law attorney, approx. 400-500€ cost. Further points may arise that cost more than 50K€. What you negotiate into an offer is not important, the contract decides. I would ask the general contractor what his KFW success rate is, and what caused the failure. These could have been the wishes of the builders.

I actually assumed that during the planning process all necessary facts about the property and house are known or become known and that the fulfillment can then be calculated cleanly.

That's how it is!

The surcharges for the Energy Saving Ordinance/Building Energy Act to KfW55 or from Energy Saving Ordinance/Building Energy Act to KfW40 are each listed individually as optional surcharge items,

These are offer items. If you decide on an item regarding improvement of the Energy Saving Ordinance and also sign the contract "House Construction KfW 55", then the general contractor owes you this service. Energy consultants and calculations are priced in. You pay for them together. But if you absolutely want only large north-facing windows and consciously want to do without south-facing windows, you should consider what kind of house you basically want.

you write offer, what matters is what is stated in the contract. If points in the offer already bother you, have the contract draft reviewed by a construction law attorney, costs around 400-500€. Further points may arise that cost more than 50,000€. What you negotiate into an offer is not important, the contract decides. I would ask the general contractor how high his KFW success rate is, and why it failed. These could have been wishes of the builders.

I would not build a KFW 55 house with any general contractor if he cannot guarantee it. It seems experience and knowledge are lacking in this area.

Interesting discussion on this topic.

If you absolutely only want large north-facing windows and deliberately want to do without south-facing windows, you should consider what kind of house you fundamentally want.

The general contractor has certainly included the surcharges for the KFW levels in his offer. KFW40 with large north-facing windows is technically usually feasible, but certainly with the extra money he wants. You can calculate all of that, and capable general contractors know what they have to do so that the energy consultant doesn't find fault with them afterward.

I would discuss the general contractor’s approach directly with him again and make it clear that this would be a definite exclusion criterion.

I would not build a kfw 55 house with any general contractor if they cannot guarantee it. It seems that the experience and knowledge are lacking in this area.

I think this is almost the relevant point. The statement feels to me like "We build the way we always do. If that doesn't suit you, not our problem." Hopefully, the rest of your house fits their standard regarding special requests, plot, etc.