Single-family house or semi-detached house?

I think we are misunderstanding each other: I do not consider the semi-detached house bad per se, but only in this specific case because of the "handtuchgrundstück" the dilemma arises to either coordinate or build a "Kegelbahn". Specifically, the comfortable choice of being able to arrange a double carport and/or a breakfast terrace by boundary construction is not present here. Rather "kill or cure". So, game-theoretically close to the prisoner's dilemma

You seem to have little knowledge of house construction in metropolitan areas. A 13*40m plot IN DÜSSELDORF is anything but a "handtuchgrundstück". That's over 500m² and in a good location quite a luxury that is often paid for with a hefty half a million euros. In metropolitan areas, terraced houses with 5m plot widths are indeed realized, and people can live a dignified existence in them as well. Every capable architect can also meet the needs of a family in a 7m wide single-family house; this is much closer to the standard in densely populated areas than the 12*15m one-and-a-half-story houses in the sticks. @OP: I would also tend towards a semi-detached house on this plot but would not fully utilize the building window widthwise, rather leave the gained space open. I would make the plans COMPLETELY independent of the neighbor. The technical necessities are then to be coordinated between the two planners; nothing else remains anyway.

You seem to have little knowledge of house building in metropolitan areas.

Yes, but I would never want to use it.

A 13*40m plot IN DÜSSELDORF is anything but a towel plot.

A towel is a towel everywhere - not only in London, but now also in Dortmund-North.

In metropolitan areas, terraced houses with 5m plot widths are certainly realized, and people can lead a dignified existence in them. Any capable architect can also meet the needs of a family in a 7m wide single-family house,

The architects in Tokyo who manage this on 3m "wide" plots (and those are short too, more like pillows than towels) have my greatest respect. But as much as one in Düsseldorf may rejoice over the relatively larger size in comparison, absolutely speaking a three-room floor is not more spacious.