Last minute bathroom redesign

Regarding your question about the size: it should be 30cm...

yes, for the shaft itself, the masonry in front will certainly take up 40cm, but I mean the height ;)

I just don't know if that fits, no idea how big such a flap needs to be.

I thought you meant the flap? Hence the 30x30cm...

Oh, I see, yes 30x30 I think is good ;)

but as I said, I meant the height, I don't know how much space the "technology" takes up in the "current" WCs

I think the stud frame is 70cm... but I don't know. Then it would fit? I would place the toilet behind this chute and like an extra deep shelf. Chute flap then from above, flush button from the front, then you still have some of the shelf. Or could the shaft be moved a bit? Towards the bathtub?

I would place the toilet behind this chute

that is also a funny idea, the question is how the planning in the floors below can handle that, for that floor plans of all levels would be necessary ;)

that is also a funny idea, the question is how the planning in the floors below copes with it, for that floor plans of all levels would be required ;)

Oops, that of course doesn't work. I mean, BEFORE that.
The toilet system would have to be designed in front of the chute system.
So the extension with the chute shaft on the wall, and then put the "toilet" system in front of it :)
But you could also discuss that with the plumber.