The stair steps are all different heights

Cast concrete stairs always fluctuate a bit in height. They are rarely ready for covering.

The “first step” with 12cm is not a step at all, since the floor construction is still missing.
The other steps fluctuate within a 3cm interval, which the tiler compensates for.
The 24cm at the top would need to be checked again to see exactly what happened there.

Why is the first step not a step?
The screed is already in place, so it will be a completely normal step. Or did I misunderstand you?

I have also already spoken to a tiler who could level it out, but I will still first address it with the builder to see how they envisaged it. Especially because of the first step and the landing, there are a few more centimeters to compensate for...

The "first step" with 12cm is not a step at all, because the floor structure is still missing. The other steps vary within an interval of 3cm, which the tiler compensates for. The 24cm at the top should be looked at again to see exactly what happened there.

I would almost bet that the 6cm surplus at the top is missing at the bottom on the first step. Because the screed is probably already in place ... and the floor covering is missing everywhere, whether at the bottom, on the stairs, or at the top. Therefore, all steps should ideally correspond, on average, to the targeted 18.4cm riser height of the stair design.

Screed is already done,

Hello

that's correct, the screed is laid.
That means the entire staircase has to be rebuilt by a screed installer.
That will be the cheapest and most reasonable solution.
The costs should be borne by whoever messed up the staircase.

Steven

That's also how I see it: what is too much at the top is correspondingly too little at the bottom. Somewhere along the line, probably a wrong height measurement was taken, so the staircase basically already sits 6 cm too low. The deviations between the other steps shouldn't be there either, but that might still be fixable.

The same thing happened to my best friend with the shell builder; the staircase to the attic was mistakenly cast using the measurements for the staircase to the basement. However, the shell builder corrected this, and they have a very good relationship, so both sides took it "with humor." On request, he would have also torn the staircase out again!

And now here it comes, who determined the height of the screed? Does it fit?

Too high at the bottom? And too low at the top.. under the stair steps 1cm is not much but it can be better.

.. the steps are 17-20 cm ... I also find that quite weak