Backfilling excavation for the base slab

Good day,

we have purchased an old house with a basement, which is currently being demolished.
A single-family house without a basement is to be built on the property afterwards, but offset from the old house.
About 3.5x9m of the new house is to be built where the old house including basement stood (see attached picture).

My question is now, since the old house had a full basement (10x10x2.5m), how this construction pit should be filled after the demolition for the foundation slab.

Best regards
Sven

Welcome to the forum,

had the same done by her contractor.
In any case, the floor beneath the base plate must have the prescribed strength and load-bearing capacity.

The old hole must be filled. For that, one needs to acquire the appropriate material.

Now I wonder how much more the new house with a basement would cost if the excavation of the new house were to be placed directly into the hole of the old house.

You would then also have a basement where the washing machine, freezer, etc. can find a place, or a hobby room can be accommodated.
We could not manage without a basement.

Welcome to the forum, the same was done by through her general contractor. In any case, the ground beneath the floor slab must have a prescribed strength and load-bearing capacity.

Although we only had a partial basement, it was filled and compacted layer by layer with a special construction soil. Even under the non-basement part of the old house, the soil had to be replaced to a depth of about 1 meter because there was only non-load-bearing sand there. This construction soil was also used there. The builder did all of this. Before pouring the floor slab, the strength was checked again. However, I would make sure that this construction soil is applied only where the new house is to be built (plus 50 cm around), if technically possible. At least in the upper layer, because nothing grows on this soil. It is so firm that I would not want more of it on the property than necessary.