How high does the house rise - how high to pile up earth (cellar depth low)?

I respect you for sticking around in this thread. I think that's great!

...from my point of view, he is not obligated here to critically evaluate the client's concept at all (how the client intends to perform their own contributions).

If he created the draft for that, then yes, he is. Who else? The original poster is obviously not capable of assessing and averting the faulty planning as a layperson. The question in my opinion is rather: Does this count as faulty planning? I would say yes.

No matter how you turn and twist it: the general contractor is not partly to blame for the backwards foolishness of the original poster.

I don't see it that way, see above.

I would probably actually build a terrace (use the space underneath as a carport, bicycle shed, or something similar) and then I would look at how you can get another door into the "basement." You just have to think "extravagantly" now and make the best of it.

If you have been very satisfied with the architect and GU so far, you can also try to speak very constructively and openly with them and share your concern. Maybe they have good ideas with their on-site knowledge about how to design it. The solution could, for example, be to fill up only 1.50 and place three steps directly in front of the terrace doors. Then you are only 80 cm above the general ground level. That is not so much.

The problems make sense. How can one prevent this here? Is there a solution that the landscape gardener has to implement in order not to have to worry?

An embankment about 1 meter high is not uncommon, meaning this problem occurs many times over, right?

The soil that would be given for backfilling is clayey. Would clay be advantageous here or rather disadvantageous?

Either slope very gently or use well-draining soil, e.g. gravel/crushed sand mixtures. These can also be compacted well. Clayey soil poorly absorbs water and tends to become muddy. If you also have that as a subsoil, you need to be extra careful with the drainage of the fill. You can use the free clay to build a small embankment for a hedge. I wouldn't build a terrace on that.

The problem is, any solution will cost you a lot. You will now need someone for advice who 100% represents your interests. The architect/contractor certainly does not, as they should have a) recommended a lift system and b) preserved as much existing soil as possible. I would look for an expert here. Ultimately, there are several options: solve it through the outdoor facilities (fills, retaining walls), convert the basement to ground floor, or take a mix of outdoor facilities and stairs.

I am not done reading yet, but distant relatives had a similar planning error. They wanted to have level access from the ground floor to the garden and the terrace on the slope and wanted to fill up the difference. The basement was planned without windows at that spot. The gardener and landscaper came and dismissed the idea. Too steep, not stable. I believe windows were eventually installed in the basement afterwards.