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

Why not? The vanity cabinet just shouldn’t be that deep. The idea is to make the vanity wall only as high as the washbasin cover just barely extends over this wall. And the part of the wall for the WC will then be a bit higher and deeper. You won’t see the part from the washbasin then. Otherwise, maybe a floor plan sketch of this bathroom, then it might be explained to you better.

The 60 cm washbasin is being replaced with an 80 cm washbasin cabinet measuring 82x49x60. The pre-wall will be 10 cm.

This would leave a passage of about 85 cm.

And how is this washbasin (T49 cm?) covered? Maybe a picture of the washbasin?

The pre-wall in the washbasin area doesn't have to be that high (how high should it be at the WC?).

By the way, the drawing shows about 20 cm pre-wall depth:

Do you really need this 10cm of space? I'm not a fan of corners and niches because, from experience, dirt always accumulates there, and I can't imagine it being nice to have to wipe out the resulting corner between the toilet and the washbasin separately when mopping the floor (which really only works well if you kneel down and do it by hand; otherwise, this corner always remains unwiped). So for the sake of the housewife and to combat the dirt that likes to settle in such niches, I would make a continuous 20cm pre-wall. By the way, we have extra narrow cisterns, and our pre-wall partitions are not 20cm. I can gladly measure at home how deep they are, but I would estimate around 10-12cm. So that would be an option too, wouldn’t it?

I also do not like this fundamental offset at all.

By the way, we have extra slim flush tanks and our prefabricated walls are less than 20cm. I can gladly measure at home how deep they are, but I estimate about 10-12cm. So that would be an option, right?

Either we make a continuous 20cm prefabricated wall or the 10cm one. However, with the slim version, as probably in your case, a different prefabricated installation including an above-mounted actuator button is required. This combination then costs over 600 € net extra. I find that quite steep.

Either we make a continuous 20cm pre-wall or the 10cm one. However, with the narrow version, as probably also in your case, a different pre-wall installation including an upper flush plate is required. This combo then costs over 600 € net extra. That's quite hefty, in my opinion.

Look at it this way: 1.9m x 0.1m = 0.19 m2 of additional official living space. At, say, 2200 Euro/m2, that corresponds to 440 Euro plus. a wider passage and a more pleasing appearance.