Offer earthworks foundation slab

75m3 of soil will be removed. Now take the length L and width B of your house and add 60cm each for the working space (minimum). Then you should be able to calculate the depth T that will be excavated: T = 75m3 / ((L+0.6)m * (B+0.6)m). If this T is now within the fill, then you are dealing with the expensive option.

You cannot use the material entirely for the backfill. But you could partially mix it with the backfill sand or create a level difference in the garden.

The information is of course only a preliminary guideline. Have the company confirm up to which depth they want to excavate. And as a precaution, calculate with the expensive option.

All right...thank you very much for the detailed explanation.