Make an external staircase to the basement from concrete?

Hello, I have a cellar door to the outside in my house. Here I still need to construct an external staircase to the cellar. I thought that a foundation for the cellar stair could be concreted and a precast staircase from the concrete plant could be installed on it. Now, the situation is that the sewer connection of the house runs under the stairs or the floor slab. Do I need to consider anything here, or can I simply concrete over it, meaning a blinding layer then cement on top. There is also a branch coming off the sewer connection there, which is supposed to be embedded as a drain into the foundation.

Aha, ...and what is in the drainage plan? How is it supposed to be protected against backflow? Is it even allowed to discharge rainwater into the sewer? That must be clarified first, then further considerations can be made!

In the drainage plan, a floor drain is drawn there, I can't direct the water anywhere else anyway, the rainwater is supposed to be diverted into a cistern, it is from the roof as well, but it is not possible from the basement because the cistern is located significantly higher than the floor drain. The backflow valve is positioned further forward towards the street in front of the cistern.

Since the staircase will probably settle a bit, make sure that the drain pipe is not firmly embedded in concrete. Especially leave 2-4 cm of space above the pipe.

From a drainage perspective not well solved, ...does the cistern have an overflow, ...into the sewer? Would a covering of the basement entrance be possible? How far along are you with the construction? Was the basement entrance not taken into account in the floor slab? Then foundations! Layer water or groundwater? If neither can occur, then I would rather accept settlements than create a connection with the basement wall. But why do you want to install prefabricated stairs? Have you considered how they should be backfilled? :-? Furthermore, the earth side also needs to be retained. It would be easier to pour concrete on-site, ... and certainly cheaper as well. Has the stair price been calculated including transport and crane? Think it over again? Shafts, foundations, and retaining walls have to be done on-site anyway, so why a prefabricated staircase if this increases the risk of additional settlements on the upper foundation because it cannot be backfilled and compacted properly.

The cistern has an overflow into the sewer. The thing is also that our house is located on a slope. The cistern is situated a bit higher on the slope side, as there is a connection to the heating room there because of the domestic water system. I have also thought about a roof covering, which could then be connected to the roof drainage. But I still wanted a floor drain in front of the cellar entrance door. We are close to moving in with the construction. The staircase was not considered in the floor slab. Many have already complained about that. I had to have it excavated further afterwards. We are building with a property developer and he only did what was commissioned. He could have at least offered to do it at the same time; I just don't think of everything either. I did not want a connection to the cellar wall, as otherwise cracks could possibly occur in the wall due to settling. I wanted a prefabricated staircase as it was recommended to me because the surface is supposed to be better than that of cast stairs. But I have not had an offer made yet. Surely, when you look at it that way, a cast staircase is probably better. I will ask about it.