A real two-family house with similarly large living space

So correct and not pseudo.
Nullbarriere helps with planning.
Hanse Haus has a barrier-free bungalow - it even works somewhat. Bungalow 133. Instead of a storage room, it has stairs.
Take the floor plan and draw in all desired or existing furniture to scale. Use a compass and check if the turning circles are given. Without measurements, we can't say much about it.
Actually, you need a bungalow.

Your budget is set too low.

Attic would be the floor above the stairs, that's enough with a headroom of 2.1m

No craftsman will build that for you.., it also won't get any approval. Sorry, but building is not a wish concert where you can just chat everything that doesn't work nice. Your calculations don't work either: Basement with living spaces 4! stairs Elaborate expensive mansard roof Dormers Light wells Loggia Kfw40

Yes, I did not draw in the roof slopes

In a sketch, not drawing them is forgiven. However, you ignored them.., attics have the property of becoming smaller and also no longer having exterior walls. You are wrong about your cardinal directions ... may I ask how old you are? I consider this somewhat "student creativity" :D Ps about 2-3 steps can be built over at most. Without measurements, you can't even determine the length of your stairs...

I also consider a semi-detached house to be sensible - and not excluded either. A tower made of two maisonettes would not occur to me. I would recommend linking "KfW40" and "kein Fertighaus" more with OR than with AND. The timber frame panel builders offer that much more often than the solid builders. The internal stairs do not work, and overall the design can be described as a "staircase house" in my opinion. Is "Budget" actually supposed to mean the selling price per apartment?

And, DG. max. 210 cm ceiling height ... you’re not serious about that - are you?

I consider this somewhat a case of youthful slight fantasies :D

And, DG. max. 210 cm ceiling height ... you’re not serious about that, are you?

I would also suggest a semi-detached house. But drastically slimmed down. 2* 140 m2 should be enough. Only the budget probably still not. Is there a site plan with dimensions? I ask again [Bebauungsplan] or [34er]?