Ok construction expert, everything is correct, understandable so far.
But we all agree that the application or, for that matter, the tapping of funding from the KFW 153 housing program has been defined very restrictively and precisely: amount x per created housing unit (WEH).
It really does not matter if the original poster goes to the bank and says, "now I’m tapping double because I might plan another housing unit in 20 years." He will only get a friendly smile in return.
Therefore, honestly, I don’t understand the fuss. Of course, I don’t know how intensive a building acceptance by the responsible building authorities is, Wiki says spot check, which is obviously bad for such things.