That is correctly analyzed and the right building gap.
Aha! But I already mentioned that.
2012 has no /19, I suspect the building gap ("2019 West" would not be an inappropriate designation) between Ludwigsburger Straße 10/1 and Hartwaldstraße 1/4 is meant:
Namely here:
According to my research, there is a house there, namely with an eastern extension, veranda or something like that, so not suitable as a semi-detached house.
Who wants to sell you the house? In my view, the builder at the time bought and built the semi-detached house plot for himself and his single-family house. That’s possible. But there should be windows on the side if there is a veranda on the east?!