Large office or a separate apartment?

In old age, you would still be more flexible and could, for example, offer separate living space to a caregiver (instead of moving directly into a nursing home).

Of course, you can keep all that in mind, but it restricts you today, either through significant additional costs or limitations in planning the current, actual living area, and in the end, you have built something that you might not need in that way or that doesn’t fit because life changes otherwise.
You can absolutely do it, but in my opinion, it should go hand in hand with a concrete life plan that reflects the actual processes. Simply building space just so you might somehow use it later would not be my approach for the reasons mentioned above.
Based on your description, a spacious office with two separate workstations could also be a solution, provided that the activities/the interaction allow it.

Of course, everyone who thinks that you don't necessarily have to plan only for the future is right. It would be nonsense to build a "forced" granny flat just because of the subsidy at any cost. I just thought, since I need an office anyway, and a guest WC downstairs anyway, then what speaks against planning a shower, a kitchenette, and a second exit door... I will talk to the architect. Maybe something practical can still be planned... I will get back to you here when I get the go-ahead for the specific building plot. Thanks for your tips.

if I need an office anyway, a guest toilet downstairs anyway, then what speaks against planning a shower, a kitchenette and a second exit door.

Of course you can do that, and if you like what the planner designs, then definitely. In the end, you can make "everything" sound good or bad, so it has to fit for you. Nevertheless, critical comments are important, but ultimately you decide yourself. Possibly, you should already consider a future separation regarding electricity, heating, water, etc., in case it is rented out.

In our apartment building (same municipality, built in 2012) there are tandem parking spaces in the underground garage. The cars (owned by the same apartment owner) are parked one behind the other and also block each other's exit. Wouldn't it be a similar situation?

You should ask the municipality how they handle it. In the end, they have to approve it anyway.
I basically only know tandem parking spaces stacked vertically, and that is already a pretty bad compromise since you can't just get in and drive off if the car in front is parked wrong. Cars parked one behind the other that completely block the other are only really "useful" when it comes to meeting a parking space requirement for an apartment that is never occupied.

reads interesting, may I ask what became of it?

sounds interesting, may I ask what became of it?

I'll throw the question back ;-)
https://www.hausbau-forum.de/threads/kleines-grundstueck-passt-uns-das.17794/page-2