Excavation for the basement, your opinion on the cubic meters

You can basically only dispose of "bad" clay. What do you want to use it for? It cannot be compacted, so nothing will work where a terrace or paving is supposed to go. If you have a lot of lawn area, then something might go there. But even there, humus is more suitable. There will definitely be a lot left over.

We have no clay.
Our soil is divided into two (three with topsoil) layers.
Layer 2 can be used both for the "base" of piled-up areas and for terrain modeling if you want to model beyond the planum.

Layer 1 cannot be used as a base, but is good for modeling.

Both, base and modeling will take place on our property, so both soil types could be used.
In practice, it now means actually looking at how much of what is used. But more important is that there is enough space for storage and work areas. That has priority.

So it is not necessarily a matter of
"There has to be something removed because there is way too much excavation and you can't use it anyway"
but
"There has to be something removed because there is too little space for storage & simultaneous work."

It may also turn out that everything fits. Then everything is wonderful and we are happy. In that case, the permission for storage next to the property would have been made for nothing.