Is a granny flat necessarily an (own) residential unit?

A granny flat is a living unit, yes! By the way, I agree with the mentioned "waste of space," but I call it missing the point if one argues with this:

gerade zur späteren Betreuung im Alter)

The argument is flawed: if you need care 24/7, then you are better off in a nursing home near a hospital. Care by relatives is usually limited to a few weeks in terms of time and does not require a granny flat. It could also be a guest room or, if care is not 24/7, living in one's own apartment. However: the subsidy still tempts people to want to declare a granny flat. You have been looking for a plot for almost 10 years… I remember.

The term "Einliegerwohnung" is, in my opinion, not legally defined and therefore unsuitable for conducting the discussion. The question is and remains whether there is a second residential unit or not, and there are requirements or characteristics for that.

The term "Einliegerwohnung" is in my opinion not legally defined

Correct. But the term "Wohneinheit" And a so-called Einliegerwohnung must have everything that a Wohneinheit must have. (For running an independent household: separate entrance, toilet and bathroom, etc.) Whether one calls it Einliegerwohnung out of habit (meaning a living unit inside a house) or apartment or Wohneinheit. Wohneinheit is a broader term under which apartments, houses, etc. are subordinated. And if only one Wohneinheit is allowed within a semi-detached house, then that is clearly expressed. There is no interpretation.

The question is, do the TS want a residential unit that is eligible for funding and actually and honestly only for that reason. Then it simply won't work there. If they want, for whatever reasons, that a non-household person can run their own household in their own protected rooms, then that would probably be possible in practice.

The household help would primarily have been an argument to be able to achieve a granny flat (yes, with the corresponding KfW designation). But if the development plan ultimately shoots that down, then the whole thing is done for again.

the household help would have primarily been an argument for a granny flat (yes, with the appropriate KfW designation)

well .. grabbing subsidies at all costs, but then maybe complaining about taxes .. that's kind of a general trend right now. And that is very very sad.