KFW 153, second residential unit

That is not an argument. It is still a room door and not an apartment door. I still consider the spatial separation of the stairwell necessary. Furthermore, it must be a second main entrance door, not a side entrance. Because the latter presupposes that there is a main entrance (via the stairwell).

Maybe I am mixing up terminology?
The granny flat is (in the case of rental) EXCLUSIVELY accessed via the separate entrance. This is located directly in the basement and leads into a small hallway that belongs directly to the granny flat...

Regarding the parking spaces, I would have to find out; there is no information about this in the development plan.

Then calculate at least 1.5 parking spaces per apartment, it is increasingly tending towards 2 parking spaces per apartment.

Why is there a door there if it is not supposed to be used?

Because we want to use the rooms as offside rooms when they are not rented. The door would then probably be closed with drywall in the case of rental.