Which shelf in the shower in the new building

I didn't know that until now. Were they installed by the tiler?

Yes, they were. (Minimum characters written)

because of the shower niche the developer says that the walls are too thin to make the niche without going through. The walls in the shower face the staircase and the semi-detached neighbor. I think we will choose an option to tile into the corners. What would you say, should we retrofit it ourselves later or have the tiler do it directly?

I have to be honest: if I had drywall in the shower, I would know that. What I mean by that is: there are also shell constructions where such a thing is out of the question. There are solid walls and that's it. Especially if you build only about 130 sqm or so – there is rarely space for that.

because of the shower niche, the builder says the walls are too thin to make the niche without going through. The walls in the shower face the stairwell and the semi-detached neighbor

, you were obviously right. That is of course a valid argument. I only had drywall stored in my mind, which was obviously wrong. Although you can of course also build a niche in a wall (11.5 cm?) towards the stairwell. With a lintel and an in-wall box. But that would probably be disproportionately expensive.

I think we will choose an option that tiles into the corners. what would you say, should we retrofit that ourselves later or have the tiler do it directly?

From the tiler. You can never get it as clean and stable yourself afterwards unless you retile both walls.

Because of the shower niche, the developer says the walls are too thin to make the niche without going through.
The walls in the shower lead to the stairwell and to the neighboring semi-detached house.

I think we will choose an option that tiles into the corners.

Where should the showerhead go, on the wall towards the neighbor or towards the stairwell? How thick are these walls each?

Could you please show us the floor plan of the bathroom and the adjoining walls? Ideally with dimensions.

Is it really a developer or a construction company? In other words: do you already own the property?

what would you say, should we retrofit it ourselves later or have the tiler do it directly?

Definitely from the tiler, otherwise you'll have to knock it off again. The system is integrated into the wall.

The walls in the shower face the stairwell and the semi-detached neighbor

I really have to marvel at the operas some OPs talk about over countless posts, where ONE picture would have explained everything.