Roof design / Use of 2nd floor mandatory with 2 full floors

If you will have the room upstairs one way or another, then it might be worth considering making the basement smaller or skipping it? I think you still need to work on your room concept. Which rooms do we need, where do they have to be located (or can they be somewhere else), and how big should they be? Then you look at how to distribute these rooms across the respective floors.

The basement is quite important to me because I need a workshop for my hobby. Therefore, painting is out of the question. One could consider turning the full basement into a partial basement. However, it is still undecided whether the existing vaulted cellar should be preserved or not.

A basement is also a living space if on average the basement protrudes more than 1.40 meters above the ground... Otherwise, one can consider designing the basement as a living floor with a slope and direct access from the basement to the garden, also with a workshop as an ancillary room. If this is not desired, then still plan the basement as a living space, for example an office in the basement, the attic with knee wall of 1.80 and 32 degree roof pitch... the attic will not be a living space, and very little attic space will remain... with 32 degrees you can also partially open the rooms upstairs to the gable.

Best regards in brief

The OP raised the question about the roof pitch and I simply recommended that he first consider the intended use and then determine the pitch.

I also think that is correct, only: everything below 30° according to his taste would unfortunately be excluded by the development plan here. Only from 30° is it possible here - and that would, in my opinion, also be roughly the upper limit for open roof undersides without making the rooms look "tighter."

It is also still under consideration whether the existing vaulted cellar should be preserved or not.

I haven't read about that so far (?)

Our development plan provides for 2 full stories with a gable roof with >30° pitch.

Is the Roman II surrounded by a circle?

The plot is almost completely flat, so no hillside location.

The vaulted cellar should not be considered for the time being, as I currently assume that it will disappear. I just have it in mind and want to ask the architect about it at the next opportunity.
We do not want the raised ground floor (is that the correct term?) for various reasons.

That means the upper floor with a knee wall of 1.80 m and partial opening of the roof would definitely be an option.

Yes, the Roman II is enclosed by a circle.