A simple way to ensure the two volumes line up is to start off correctly.
Take the EXTG group and add a skirt using the tool’s utility.
Next copy that group and rename it Proposed - move one of them to one side temporarily.
Edit that Proposed groups and it automatically becomes unique and separated from its sibling.
As it has a matching skirt there’s no need to add it again…
Stamp you house on the surface and now you have two volumes - EXTG & Proposed - move one of them so a corner of their skirts align.
Now select and process the two groups using CutNfill - the EXTG name is needed to identify which is which…
As said before… don’t make mm adjustments to the Proposed surface as these can result in very tiny wedges <1/1000" which fail to remain solid in the solid-subtraction processing and report zero volumes… In real life you don’t worry about a few mm in a site surface when calculating cut or fill, as simply walking over it will affect it anyway !