or we close the exit downwards.
If the basement is not to be included in the thermal envelope, then definitely. The separating door can be at the top or bottom. You can’t see around the landing corner anyway, so I might possibly include the exit visually towards the top. Or you let the basement exit at the top appear like a storage room door.
Another dividing line wouldn’t make much sense
Certainly not stepped multiple times, but maybe slanted.
But I don’t know if the variant with 2 full floors and attic as an expansion reserve for the higher floor wouldn’t fail due to the max FH of 9.50 m...?
I don’t know either, that has to be calculated specifically.
Would it look stupid to put the entrance at the front on one half and on the side on the other?
No, that’s actually quite common for "classic" semi-detached houses on corner plots, and with unequal halves that would even be my "first choice". They usually aren’t suitable for mirrored cloning of the layout anyway (instead, for the north half, the west side is often taken as the preferred side).
We haven’t even talked about the other half yet.
Indirectly yes: at first, equal-sized halves were planned, which should only differ in the standard. Therefore, I would already have expected a mirror clone there out of “imaginative economy.”