We think the extension was built flat on the ground, except for the stairs leading down to the original 1925 basement.
The exterior of the extension is old, cracked and falling off in chunks insul-brick.
We've noticed a small (6" by 8") damp patch on the wall of the lower stairs which lead from the mud room to the original basement.
We were thinking that the insul-brick needs to be replaced, but at the same time we should have waterproofing done below ground level of the extension. We suspect, given the age of the extension, this was never done.
We think the damp spot on the wall is just the wet solid earth behind the wall of the extension where it is below ground. We have no dampness at all in the original basement area.