Basement floor construction

Hello, I have the following question: I want to rebuild a basement floor. The room is intended to be used as an office later and will therefore be heated. I have now made a hole to see how the existing floor is constructed. In doing so, I found that starting from the height of the new floor, there is about 23 cm of foundation under the walls. Below that is sand. The foundation is also embedded in the sand. Now the first question. Am I allowed to remove these 23 cm in which the foundation is embedded? I am clear that I must not go deeper than the 23 cm. But am I allowed to remove the 23 cm up to the depth at which the foundation begins??? If yes, the new floor construction would begin from there. If the new floor construction is less than 23 cm, I would simply leave the corresponding amount of sand. Then I would not have to remove so much. That would also be the subsoil to be assumed for the new construction. I have also spoken to a screed installer about this. He would build the floor as follows: - Concrete screed 6 cm - Styrodur 6-8 cm - Foil - Sand Would this construction be acceptable? Or should gravel or something similar be placed on the sand first? I would be very grateful for answers. Best regards mat1984