So the basement builder only offered to produce the execution plans. The house builder then created the basement design, but the basement builder said (I’m exaggerating now) that it is not practical and that he cannot or does not want to create working drawings based on it. Now both talked on the phone (you stayed out of it) and decided that the basement builder will create the basement design. Have you spoken with the house builder about whether he will still fully charge for the basement design on his side? He would be fully within his rights, but maybe a compromise can be found somewhere in the middle? I didn’t quite understand what this thermal envelope and your heating is about, but if you don’t have any “trick attempts” to blame yourself for here, I would try to negotiate a cost compromise with the house builder. Either the basement builder had a valid reason to reject the plans so strongly, or the fault actually lies with you?! It always takes two, and the truth is known to lie somewhere in the middle ;-)
*Edit:
You don’t have to argue, but a lot can be clarified in 6 months
So again: We bought a ready-to-move-in prefab house WITHOUT a base slab.
Since the house manufacturer does not offer basements, but we need one due to the sloping site, we ordered one from a renowned prefab basement manufacturer (an unheated basement where storage rooms, workshop, utility room, HAR and the heating are to be accommodated).
The house is therefore thermally separated from the house.
The first point of contention was that the basement builder filed an objection against the heat demand calculation made by the house structural engineer. This was resolved in the meantime by a third-party neutral energy consultant I engaged... both were right, both approaches could have been chosen.
Secondly, it turned out that the execution plan for the basement named in the contract is not the execution plan as we understood it. I am still very surprised by the contractual formulations, however this basement builder constructs many basements for prefab houses and so far it seems everyone has understood it in their own sense.
After a long back and forth, he has now agreed to prepare the required execution plan himself, billing based on effort.
I will have the contract formulations legally reviewed again... but what good does a dispute with the basement builder do me now?
Cancelling the contract and commissioning a new basement builder will be more expensive than simply paying him for his expenses.
So just to say this again, I am not completely inexperienced in real estate. I have for years commissioned apartment renovations from the commercial side and also supervised on site, I am a real estate economist, but the subject of basements has now hit me quite unexpectedly.
And to also state this very subjectively from my perception: Professionally, I never felt that any craftsman did not take me seriously. And when I built my first house as a single mother myself (also a timber prefab house), all companies cooperated well with me. But now, here in 2023, when my husband and I are building a house together, he, as a police officer, has no knowledge of these topics at all, now the individual companies are starting to want to “talk to THE homeowner”… That’s annoying!