Is it sensible for the expert to inspect the base plate?

Thanks, but I’m being told that those from the shower also go into a pre-wall. Everything that faces the neighbors, the wall (it is a corner townhouse after all), must go into a pre-wall, and everything that would only be my wall, basically the wall to my children's room, could be inside. But the bathroom plumber doesn’t want to put anything in the wall, which I found strange; later it turned out it’s more work, apparently for the same price!!! We are currently feeling very poorly advised and tricked and have already complained.

but it is also strange, I have my own wall towards the neighbor... we do not share the walls

It's a matter of taste. But in moderation, I find them quite practical. You don't have to fill them up completely. Especially around the sink, you quickly end up with too little space for everyday items, and before I hang up some kind of Alibert like in the 70s? Well, I really liked our pre-wall in the old house. In the new one, we'll have the pre-wall "to the side," so not directly in front of the washbasins, but on the adjacent wall, so we can still make use of the corner a bit. And in the shower toilet, there will be a pre-wall in front of the washbasin, which we'll highlight even more with alternative tiles. We chose the BÄRWOLF Ambience Spatolato Blossom Grey for this, which we'll mix with matching plain tiles. Pretty much exactly like in their catalog. I don't even want to think about how much extra money we'll pay just for this "feature pre-wall," but well, I just love floral kitsch. =)

I want to point out one thing. If you have water pipes "chased" into the wall, the wall is thinner at that spot. So in our case, the pipes for the bathtub run in a wall to the children's room, and despite insulation you can hear it loudly in the children's room. So I wasn't allowed to take a bath once the child was in bed. Now we are switching the rooms, but I wish there was a partition wall there.