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

A constructive suggestion: pile up 1m of earth and then secure it with L-shaped stones. That should be quite feasible. Plaster the house on the spots that then stick out and create a staircase from the living room and entrance area to the garden. A staircase with one meter should not be too disturbing.

And I hope you were allowed to build two full stories, because if the development plan prescribes a single-story construction, you would have to fear a usage prohibition due to the basement height. Because with such a high basement, you would effectively have two stories.

That was to be feared. So a worst-case scenario. Honestly, I see a clear planning and consulting error here. It’s not even certain whether this still counts as a basement or already as a full story and how that is compatible with the development plan. The question that arises for me is whether the general contractor should be so easily let off the hook here. The correction of this mess will, in my opinion, cost around 70 to 100K - depending on the execution.

How should one behave now with the GU? To what extent would you see him in which responsibility?

That is the crucial question. First of all, one would have to open a big can of worms to make the general contractor aware of the problem. One could take the position that the terrace doors do not work because this elevation of the terrain was never intended and is also impossible (see water drainage, neighbor shading, etc. – here you have to get everything out that is possible and has already been mentioned). You were not / poorly advised or informed about it. He bears partial responsibility and should contribute to the costs of remedying the mishap. That would be my rough direction.