How to cut (excavate) topo with family

[ct_studio]

I am trying to cut a toposolid with a family (category column). I am trying to "excavate" rather than just slice the toposolid. 

I tried making an invisible solid within the family.

I got this error:

Does anybody have a suggestion how to accomplish this?

Thanks.

[ToanDN]

Why don't you use Excavation tool?

[pessi.symb]

how about Building pads?

or maybe create a Subregion and change the elevation?

[ct_studio]

What about dealing with a sloped floor?

I am trying to excavate to the edge of the floor only.