Main Pipeline Plan - Wastewater Drainage and Elevation Planning

But you would know where his sanitary fixtures "are concentrated" and whether he is building with a basement (where it can shift again) or without (where it would go through the slab at a quite certain point).

Yes, attached are the floor plans of the neighbor. The drawing on the site plan above should also be correct. Note the rotated cardinal direction (don't ask me why the architect did it the other way around here, it was the same one).

I assume that he will take approximately the path you have marked in green, but protruding from the base area above the "o" of "Wohnhaus II," so about 2 to 2.5 meters longer than in your drawing.

If there are 2 separate sewage pipes, the neighbor cannot lay pipes across your property to the shaft, there is no [GFL], and if his mother-in-law sinks the rag in the toilet, you still help him dig up your garden.

correct. Although a branch line to the sewage shaft is guaranteed in the KV, to my knowledge it is not registered in the land register, at least not as a building encumbrance.
that almost argues in favor of putting a shaft into the GFL after all... although, that could also be done later...

So, the construction manager agrees with my version. For wastewater, it is at least just as good, or even better. He is not worried about the elevation connection. We can stay at a depth of 1m even under the driveway, as long as it is well compacted. He also said that up to 75m of pipeline length would be okay, so with our max. 50m we are well within limits. He believes it can also be done as a DIY project, but his recommendation would be to have it done by a civil engineer. I am now getting quotes and then I will decide with my neighbor how we proceed. Basically, we still have some time for the utility connection.