Basement slab on existing cellar

Hello,
I want to build a prefabricated house on the basement of my current house. The house dates back to 1968 and has a thick concrete slab as the basement ceiling. To what extent does this have an impact on the new floor slab that will be placed on the basement so that a prefabricated house can be built on it? Does the ceiling slab also need to be demolished? Could it possibly even represent part of the floor slab? According to which dimensions/conditions, etc. should I look? Many thanks in advance. Unfortunately, I have little knowledge in this area and would like a few tips on what my next steps should be to properly approach the planning of the floor slab.
Best regards

I want to build a prefabricated house on the basement of my current house. The house dates from 1968 and has a thick concrete slab as the basement ceiling. To what extent does this affect the new floor slab that is to be placed on the basement so that a prefabricated house can be built on it? Does the ceiling slab also have to be demolished? Can it possibly even represent part of the floor slab? According to which measurements/conditions etc. do I have to look?

What kind of house is currently on the basement, why is it to be replaced, is the size sufficient for you for the new one? Show plans of the current house, pictures wouldn't hurt either. 1968 was a good vintage ;-) How do you know that you are dealing with a "thick concrete slab"?

I unfortunately have little knowledge in this area

You don't need to either. A fish doesn't need to know how to ride a bicycle.

what my next steps should be to properly approach the planning of the foundation slab.

Planning here is more the responsibility of the structural engineer than you - go to one or an architect and work through these steps together.