Prominent staircase with different step heights

Thus, for the first 15 steps I have a height of about 17 cm, and the very top last step has a height of about 29 cm.

Is that calculated only in the loaded state? - then I would check whether it is possible to add a last step behind the last step; i.e., to attach another step that builds up on the raw floor - screed and underfloor heating would then have to start one step later. If that is not possible, the cause must first be clarified more precisely before a solution can be developed. Hopefully, you have a completion guarantee, so that the builder doesn't go bankrupt over roof and stairs being removed :-(

I think this botch job is quite unique.

So if I understand you correctly, you want to build something like this (with 15 steps):
Step 14 gets a 9cm increase
Step 13 gets a 6cm increase
Step 12 gets a 3cm increase

Doesn’t sound impossible at first.

Yes, exactly, determine the height between the upper floor floor and the attic floor. Then calculate the corresponding floor coverings and wooden step heights and determine a new step height. This step height, for example 18 cm, I would then have to divide in consideration of the tiles (2 cm, tile and adhesive) in the attic, the wooden step height (step 4 cm + 1 cm material), and the parquet (2 cm including adhesive) on the upper floor, and re-cast/level each step starting from the top.

The problem results from the fact that a build-up of 30 cm was made in the attic and a build-up of 15 cm in the upper floor. However, the staircase was not planned/installed accordingly. Thus, the styrofoam and screed are on the top step in the attic.

The problem results from the fact that a buildup of 30 cm was made in the attic floor and a buildup of 15 cm was made in the upper floor. However, the staircase was not planned/built accordingly. Thus, the Styrofoam and the screed are on the top step in the attic floor.

And who is responsible for that?

And who is responsible for that?

The property developer

Nothing helps .... you should seek advice from a specialist in interior construction. You should also have a conversation with your construction expert or architect ..... Doing it yourself is not recommended, a professional needs to handle it. Better to invest the money than live with the problem for years.