Einliegerwohnung - definition question

Can I contribute that among the reviewers, it is generally assumed that a house with a granny flat is worth about 10% less compared to a house without a granny flat

Less? As paradoxical as it sounds, that would actually be better in my case if it were worth less! ;-)

As said, it is merely a rule of thumb and nothing really tangible, since in the end "every property" has its peculiarities. Since this is also not a clearly separate granny flat, it is definitely a special case. Because the rentability is certainly limited by this. In the case of the basement, the first question is whether it is even approved living space?

When it comes to the basement, the first question is whether it is even approved living space?

According to the architect's plan, yes. The house is built into a slope so that the front side is level with the ground and has windows. However, the rooms are scattered all over the basement floor. To get to the bathroom, you would have to go through the hallway. Between two rooms on one side of the house and the other, there is a small anteroom, which in turn leads to a room where all the pipes converge, the inspection shaft is located, and the fuse box is. This means that as a landlord, you would necessarily have to have access to this room and would thereby be in the tenant's area. The house was and certainly is not intended or designed as a house with separate apartments, my parents as builders had no interest in that regard. Therefore, the question is only whether these would be separate apartments as the dear co-heirs claim.

whether they would be granny flats

Forget it. Or rather: the dear relatives can forget that.
Here you can see that it is an artificial expansion of a plain single-family house which was not built to accommodate a granny flat.

Of course, with more or less elaborate conversion measures, you can add something extra to many buildings or change their use, but here it is a single-family house with a finished attic and basement – nothing more (if your description is correct, and one can assume that since you gain no advantage from a false statement).

but here we are dealing with a single-family house with a finished attic and basement - nothing more (if your description is correct, and one can assume that, because you gain no benefit from false information)

I hope it becomes clear from my posts that I want exactly the opposite, that is, not to artificially enhance the value of the house ;-) And I thank you for the many tips, that already reassures me!

I have one more question in this context: in the neighborhood there is a similar house, similar in construction, year of build, size, etc. However, this house additionally has a completely separate apartment, which is very large and has its own entrance with its own front door. You cannot get from this apartment to the main house or vice versa from the main house to this apartment. I know what this house is worth and could draw conclusions for my own house from this. Can you roughly indicate in percent how much value increase a house receives through such an apartment?

Can one roughly specify in percent how much value increase a house receives from such an apartment?

In my opinion, no, because factors such as condition and features as well as the property (size, orientation, layout) play a role.