House construction, KFW70, approx. 150m², which heating? Gas/air-water heat pump?

You can definitely give it a try. Whether the GU agrees to it is a matter of negotiation.

Yes, it's that simple... however, you have to clarify that with your provider BEFORE signing the contract...

No, it is not as simple as you casually claim.

He is just a salesperson and wants to sell you something... so you hold the stronger position before signing...

First of all, a potential customer does not automatically hold the stronger position; there are also saleswomen who under certain conditions waive an order. On the other hand, salespeople can promise a lot – unfortunately, such colleagues exist, and whether their employers approve of these promises is another matter entirely. When individual trades are to be carried out in EL, the question of warranty always arises. It should be clear to everyone that possible defects that occur cannot always be clearly attributed to one party or another if the EL is to be implemented during the ongoing construction process. The provider who agrees to this – if of sound mind – will always demand an expert acceptance. If they are very clever, even two, and then the question of sensibility arises, because an acceptance also costs money. Rhenish greetings

And yet contracts are signed daily and houses are built with the most diverse shares of EL... the least of home builders buy a house that is completely habitable after such and such a construction period and when the GU/GÜ is finished with it...

We do it with plumbing, heating, and earthworks and I wouldn't necessarily recommend it if you have no time and no patience (the whole thing is not bad, but partly an endless running around - but in the end you know a lot about bathroom fittings - more than you ever wanted to know )

Then build with an architect

And yet contracts are concluded daily and houses are built with the most diverse shares of EL... very few homeowners buy a house that after such and such a construction period and when the general contractor/main contractor is finished with it, is completely habitable...

That is correct. However, it is the case that EL should sensibly only begin after the handover of the house, so that the question of warranty is clearly defined. That means a certain construction status is agreed upon, which the provider owes, and only after completion of these works and handover to the builder do the own contributions begin. Since in controlled residential ventilation many things interlock, I would assume a refined shell construction as part of the contract. Rhineland greetings