New single-family house in southern Germany

Storage space is important when it is USED SENSIBLY.

That's right!
That does not include an armchair that is still good: if you build properly, then you will find a place in every room where you can put a still good armchair as a (decorative) object, basically to temporarily store it, temporarily park it, decorate. Furniture does not belong in the attic. Those days are over.
There are replaceable pieces of furniture where it is not even worth carrying them up a staircase, and there are pieces of furniture that would lose their soul in the basement or attic. Then you have to give other people the opportunity to take care of the piece of furniture if you yourself don't have space for it.

The elderly who nowadays have to part with things because they have to move already die several deaths before the actual one when they have to part with dowry, old furniture, and dishes. Back then people just didn't know any better - nowadays one should act with awareness when one is ready for building a house and having their own household, and this also means questioning "old customs," why one takes them from the older generation when you find everything they do somehow "old."
https://www.hausbau-forum.de/threads/die-liste-die-jeden-bauherren-zu-interessieren-hat.34418/

Admittedly, I belong to the category of hunter and gatherer

The mirror of the soul of a chaotic person is the untidy surface in the browser -> a collection of tabs. That is also the mirror of the house... fits! :p

I sometimes wonder where such things disappear to in these new buildings without basements and attics.

At my cousin's, I suspect everything ends up in his parents' barn (a disused farm). But that's not necessarily a sustainable solution either. I definitely can't imagine building completely without storage space.

The mirror of a messy person's soul is the untidy surface in the browser -> a collection of tabs. That's also the mirror of the house... fits! :p

I permanently have about 40-50 tabs open in my browser...

This morning, a local general contractor also submitted an offer. A detailed construction description is still pending, making comparison difficult. But even if you subtract things like earthworks that are already included in the offer, the hope that it will be significantly cheaper than, for example, Weberhaus will definitely not be fulfilled. I'm a bit frustrated right now because of this. Somehow, all offers seem to come out just under €3000 per sqm, regardless of the provider. Somehow, I'm haunted right now by the statement from the HeldHaus salesperson who once predicted a required budget of €600k ("also with other providers"). We haven't quite reached that yet, but the basement is currently excluded, and in the offer from the local general contractor there is no 40+...

What is included?
How is the equipment?

I permanently have about 40-50 tabs open in the browser...

fits the attitude "what I have, I won’t give away anymore" :p

But even if you subtract things like earthworks that are already included in the offer

Do you actually read what is being written here? Earthworks are (always) on top, because the 30 cm from the contractor (GU) are (no longer) sufficient.