Thank you for all your answers!
I didn't reach our provider yesterday, maybe today, but I reached another one who scared me even more than I already was. He said that for the second residential unit, a second ventilation system is also required, and additionally the basement ceiling must be better insulated (even though the basement is already much better insulated because of KfW 40??). Our provider didn't tell us any of this (but he is also one of the very small ones, and the other one (who can't build us a granny flat at that price anyway and generally likes to complain about the competitors) which makes us extremely anxious. He also said that if one residential unit is for any reason at risk of losing the funding, we won't get any funding at all, not even for the house.
We need this funding but somehow we can't manage all this. Is there any list from KfW or BEG where it shows what has to be done for the second residential unit to receive the funding? We have already read pages of PDFs and found nothing like that.