Large office or a separate apartment?

And then the parking space requirements.

A room of 16-20m2 would be completely sufficient for us (+ WC with shower + mini kitchenette (2m2)

I consider that unrealistic ... it is better to plan 2 rooms. If someone often has to conduct conference calls or training sessions, it is very disruptive if a second person is in the room. There are some technical solutions for that, but better to plan 2 rooms as, for example, described.

Such a subsidy is nice and all, but as others have already said, where should it go? The basement is the most obvious option, but that is significantly more expensive than the subsidy if you would otherwise have built on a slab. A basement with KfW40 is very expensive anyway. And if you don't want to sit in the dark all the time, you also have to plan accordingly with light wells, etc., which is not exactly easy or cheap. It's different if you have a hillside location; then the basement comes naturally, and you can easily get daylight from one side. Plus the possible outside entrance. Then simply put a lockable door at the basement stairs and you have a granny flat that is either accessible from the living area or independently from the outside. In short, without details about the property, no one can give you anything meaningful.

What exactly do you want to know now?

What do you think about it. Whether you would think similarly in our situation? What such a granny flat would additionally cost (a study room has to be available anyway): 50k-100k?

What does the development plan specify? Usually, parking spaces are required for one residential unit, etc.

2 parking spaces per residential unit. We want a double garage anyway. I think the driveway (min 5m paved area) could be considered a double parking space ;).
Building without a basement. The 2nd residential unit would have to be on the ground floor. I know it needs a separate entrance. But is it allowed to be additionally directly connected to the main apartment (door)?
Renting to strangers is not planned. Possibly for our own children if they (and we) need more quiet, or our parents if they are no longer able to live alone...
These are all just options that are possible for us.

The building is done with an architect. I assume he can properly plan such a mini unit ;). But I want to contact the architect only after I get a commitment for a specific building plot.

In short, without details about the property, no one can give you any meaningful advice.

Yes... that's why we are already dealing with the matter now. About 40 plots are available. When applying, we have to submit a ranking list with 5 building sites. Most of the area is flat. A few of them have a slight slope (I would say 1-1.5m height difference in the building window).

We have similar plans. This means that we have declared a 16 m² office/guest room planned on the ground floor, a 4.5 m² shower room, and a 2.5 m² hallway as a separate apartment. Access is possible from the main hallway and from outside. So, we have a house with 2 living units. According to our energy consultation, there is no minimum size requirement for such a unit from KFW, and it has already been implemented this way by our architect in a few houses.

We have something similar planned. This means that we have declared a 16 m² office/guest room planned on the ground floor, a 4.5 m² shower bathroom, and a 2.5 m² hallway as a separate apartment. Access is possible from the main hallway and from outside.

Could you show the floor plan of your ground floor here (or private message)? I would be very interested.