Floor plan design of a two-family house with a long fire wall

didn’t you banish part of the building services from the house?

Correct. The entire building services of both semi-detached houses are housed in an external (half-divided) technical room. A condition for its boundary location was that no combustion source (gas, oil, pellet) is installed there. Since we are also building a passive house with KfW40+ funding as info: It is not part of the thermal envelope! That would have really been difficult. Upon request, I am happy to provide floor plans and pictures. Unfortunately, they are no longer "online". I haven’t followed the thread now, but our plots are narrow towels. Mine is only 11x28m! Boundary development (semi-detached house) was therefore sensible but in any case predetermined.

I scribbled something. Hope it’s readable

A semi-detached house

An apartment almost designed the dining area like a conservatory

a U. A semi-detached house with a small connection to get more light.
I don’t know what you are allowed to build.

Is there a reason why you dock in the west and not in the east?
Could you please integrate the building structure there? How far does it extend?

I'll say it straight: with a property, even if it is “only” a share of a two-family house, you make yourself immobile. You invest money and commit yourself to what you buy or build. Building with siblings even more so: you make a commitment. You should consider this if in the end “only” an apartment results that does not grow with you and already offers no alternative space if a child is born.
How is the rest of the family supposed to live in the ground floor apartment when there are guests? Tupper or gaming nights, football games or whatever else. Where is ironing done?
If you rent something like that, you can eventually leave, rent something else because it is too dark or too small... Here, with the obligation towards the brother, it is not easy to reorient yourself.
Personally, that would be too little for too much investment for me.

Apart from the fact that I am not in favor of siblings joining forces with their partners. But it is certainly not intended that one speaks frankly about this relationship here – everyone probably has to make their own observations or experiences that some see serious trouble threatening the nest-building.

Oh, I almost forgot: I still think that a lot has been achieved on the base area. But it remains a residential character. Maybe the project is more suitable for an architect rather than a planner of a GU.