Build a terraced end house with an additional unit (GÜ) on your own

No, our flush plate is on the front. It would have worked from above as well, but also from the front. Geberit Sigma 60

Can the false front at the washbasin not be omitted entirely? The pipes should also be able to be placed inside the wall. 20cm is, in my opinion, quite a lot for a cistern; I'm pretty sure ours are narrower.

Can you completely do without the pre-wall at the washbasin? The pipes should also be able to be laid inside the wall.

That's just a 10 cm interior wall made of aerated concrete as far as I remember - chasing it horizontally for about 50 mm drain pipe would be insane.

Can't you build the enclosure yourself? It maybe costs 40 € in materials and takes 1-2 hours and is really easy, even for those less skilled in craftsmanship. You get a nice hourly wage out of it.

...well, the plumber can rightly refuse any warranty. Or even the subsequent trade.

That's just a 10 cm interior wall made of KS AFAIR - to chase horizontally for about a 50 mm drain pipe would be madness.

On the contrary, since it probably isn't load-bearing at that thickness. You don't chase, but rather cut out. Pipe directly into the wall. Insulation wool around it, if desired, mesh over it and plaster.