A large space concept is looking for ideas

Now I had already sent the answer, I mean.

What do I hope from this thread? New ideas and impressions. Tips on how to implement our room concept without completely blowing the budget. Alternatives to prefab house manufacturers. Suggestions on how to find regional general contractors who are reliable. Google somehow doesn't come up with much.

UG:
Office about 17 sqm plus office about 35 sqm, technical/storage room 15 sqm, entrance area 15 sqm

This idea has not yet been implemented in your posted floor plans.
Try to incorporate it and then post the new floor plan here including dimensions.

Yes, the hallway is somewhat smaller in the floor plan, but otherwise it should be very similar.

If you put the technical room in the basement, your hallway will be more spacious. What will you do with the extra sqm of the utility room? Cloakroom? Larger hall? More living room or kitchen? Or make the whole house smaller?

The technical room is not actually going away. A pantry is to be created from this.

There are not 4 showers in the house.

Two on the upper floor, one (dashed) on the ground floor, and the basement plan also states that another shower bathroom is to be added there. So I also count a total of four.

Currently, we can only present offers from prefabricated house manufacturers. The two regional general contractors quoted prices that are clearly well above our budget.

I think I already said it: the "higher" price will be the "truer" one :-( That means you can now cancel (not paint, haha) or sign with the one who is cheating you with the more appetizing offer price...

The technical room is not actually going away. It is supposed to become a pantry.

Don’t plan specific room swaps: we are still far from serious plans here, so far only discussing dummies.