Build a semi-detached house alone?

That sounds intense! I would love to see it sometime.

It is probably understandable that I don’t just post the neighbor’s cabin here. I will have a look in the next few days to see if I can still find some meaningful photos. ;)

There is no requirement that the semi-detached housing must be built with identical houses, unless the _BPlan_ specifies it. Just because developers take the easy way and/or offer it to the buyer/building owner at a low cost and therefore plan a twin house, it is neither law nor prescribed.

So, I take away: if I build the semi-detached house with a basement, then it is 80% good. I will arrange the insulation of the party wall with the building company regarding the not yet existing neighbor.

No, it is not "good," but your private matter whether you want to build with a basement. If you decide "for a basement," then against the background of your neighbor's yet unknown decision, it is 80% "the safer way." The question of whether you want to keep the insulation on the shared side of the house permanently can only be decided by yourself. The building company will say, for example, "You are the client." For instance, you can make an ETICS wall on your half on that side.

Why would anyone put up with a semi-detached house, it has absolutely no advantages, especially since you don’t save anything as there is no joint project. But you have to endure all the disadvantages for years.

Because there are regions where not everyone can afford a 400sqm plot for 500k …

Because there are regions where not everyone can afford a 400sqm plot for 500k …

The plots are almost the same size. And at 500k euros, the 10,000-20,000 euros don’t make much difference.