Initially, the granny flat is used as a guest area. We lived in Switzerland for 6 years, and visitors usually stay longer upon arrival.
Granny flat also for the reason that my parents and siblings currently live opposite. Thus, there is the possibility that they might move in later or a child could possibly move into the basement during their studies.
Additionally, a 2nd building unit, meaning a 2nd KfW subsidy.
I can "somewhat" understand the first part, although visitors usually don’t actually need their own apartment.
I did not understand the second reason.
However, if money is less of an issue, then just do it!
The only thing that would bother me about the floor plan is the lack of space for dressers and shelves in the living/dining area.
But of course, that’s a matter of taste. Other than that, I find the floor plan very well thought out and excellent!