Check the thickness of the joists

[Mac_attack]

I think it is a questionable solution to create slope for drainpipes through overlay casting, regardless of the type of concrete used. It is fundamentally incorrect and means that the problems are being deferred. With precast slabs, TT-cassettes, or Kaisertype, the necessary slope is taken up on the underside of the slab in the hollow space.

That would imply a rather substantial "box" running across the kitchen below the bathroom, all the way to the laundry room.
Alternatively, making holes in the slab's beams crosswise in several places.

Overlay casting feels like a common solution? It seems that you see raised thresholds in just about every other renovated apartment bathroom.

 

[justusandersson]

Elevated bathroom floors are probably common in renovations, but likely not as castings, which is what I am opposed to. The type of beam framework you have is uncommon in residential buildings where the need for a large span is not particularly prominent. It is more common in premises for business activities. The drainage pipes have to take extra long routes to avoid crossing the framework's beams. I cannot comment on what is best in your case without floor plans.