So adding grandma is planned in about 15-20 years.
I spoke with the office and the lady was very kind, it looks as follows: the house was originally built as a single-family house, was actually reclassified as a four-family house, but if we now only want to use it with two families and there are no extra apartment doors/entrance doors with triple kitchens, there is nothing against it and I only have to provide proof if I want to add floors, not if I reduce.
By the way, it was said that they will only come to inspect if a building application for the extension of living space is submitted.
So lucky, I explicitly asked again how it looks regarding entries as a two-family house. No, it is not necessary.
But thank you very much for the help!