A real two-family house with similarly large living space

The cars are facing the street (on the short side). South is towards the street, the stairwell on the east side. Semi-detached houses are not permitted, only one semi-detached house per plot. : because I need it barrier-free for the family. I don't understand the comment "Terrace is not garden access," could you please briefly explain that. : far too little?

What do you understand by barrier-free? Without thresholds or also suitable for assistive devices?

A rooftop terrace does not replace a garden. Where do the children play? Also for adults - hammock, grill, raised bed, whirlpool. Outdoor sauna etc. A house offers more possibilities than an apartment

The staircase on the 1st floor to the attic, which theoretically protrudes into the guest room, somehow suboptimal

BOTH internal apartment staircases do not work at all. You noticed one yourself, in the attic it is with the staircase opening in the hallway... the other is in front of the kitchen sliding door. Stairs have a stairwell/opening above them so that you don’t eventually hit your head ;) you completely ignore this fact.

Mansard roof for better use of the attic

? With an eaves height of 4.30??? You also completely ignore the resulting roof slopes?!

Balcony for the other apartment

That is a roof terrace, in this case with the roof structure an integrated loggia in the roof, basically a hole in the roof, not a balcony.

I estimate around €520,000

You are completely wrong there.

What can I include in the difference of 42 m² (balcony, bay window, light wells, courtyards, terrace…)?

All the mentioned things should, to my knowledge, be included in the normal floor area number, so in the 162 sqm.

Extended floor area ratio: for garages, etc. "the floor area ratio may be exceeded up to 0.5." What is meant by this mathematically? Half of the plot area (405 m²),

405 sqm possible, subtract the actually built area of floor area ratio I from that. It should be roughly like that.

With which wall thicknesses for exterior walls (KfW 40 with brick cladding) and interior walls do we have to reckon?

You should ask the architect about that. There is nothing general. Load-bearing walls, roof shape, ... the KfW 40 standard has a big impact on price determination. The problem are living spaces in the basement. Currently, I see a 3-story building that has nothing to do with the allowed 2 full floors or a 2-story building with a mansard roof.

The cars are facing the street (on the short side). South is towards the street, the stairwell is on the east side.

Do you have two streets? One in the north and one in the south? So north is to the left on the plan? So the sun moves the other way for you if the stairwell is on the east side?

Regarding my invoice:
Basement 80k€
Roof 81k€ (Roof frame 36k€ [100€/m²], Covering 23.4k€ [65€/m²], Insulation 21.6k€ [60€/m²])
Stairs 24k€ [800€/m²]
Shell construction 120k€ [150€/m²]
--> Shell construction + Roof frame & Insulation and Basement = 260k€ --> 50% of the costs, so 520k€ for Exterior and Interior

: Both apartments have about 200 m² of garden, on the property I can push the house further back, that was too complicated for me in the program for the first draft. Barrier-free = wheelchair (yes, I know you can't get into the basement without a stairlift, etc)

: Thank you very much for answering the questions. Regarding stairs, I haven't come up with anything better so far and upstairs in the attic there would be floor above the stairs, that is enough with a headroom of 2.1m Nevertheless, I am basically not satisfied with the stairs. Yes, I did not draw in the roof slopes and also not the dormers that I would need, but I am aware of them. I couldn't manage it with the program.

Here again with the street and the entire property (please imagine a continuous grassy area) and a side view.