Hello swix112
Who are you building with? An architect or a construction company?
Either or both can give you the appropriate recommendations based on the plot data and house planning, or calculate the exact costs for you.
Basically, the following:
If you have not wanted to build a basement so far for cost reasons, then the offers for the fill material should be determined precisely and obtained immediately. After that, you will see, based on the data provided, whether the filling or a basement would ultimately be cheaper (apart from the fact that the basement would of course still be usable).
As the building expert already wrote... topsoil as fill material is definitely out of the question! That would be like putting the cream at the bottom of a nice delicious cake and then wondering why it squirts out on the sides! I think you can understand the comparison... and from now on know... that won’t work!
If you need/want to fill a level... there are always different options, which ultimately only differ in price.
You can fill the area with normal clay soil... compact it as well as possible... and/or then dig down to the old, load-bearing soil for the foundation. This strip foundation can be poured with concrete... and that’s fine.
However, the slab above will then be calculated structurally like a ceiling... it carries from one concrete foundation to the next... as if it were resting on walls.
The other option would be to create small strip foundations... above them, up to the corresponding height, either a "lost formwork" or also shuttered concrete walls... appropriately and depending on the height, sensibly..., to create. You don’t need to fill the spaces between... the concrete slab is again designed structurally as a "ceiling"... and from there come your individual floors... that is, your house.
This solves the structural issue flawlessly... but beyond a certain height, this will no longer be cheaper than a simple basement floor!
Therefore, you should have this offered by your construction professionals... even before you sign the construction contract... you should ultimately only do that if everything is clearly regulated...