"Alexander Galkin" wrote in message
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snip

The cost of excavation has to include digging next to an existing
unreinforced wall without colapsing it; excavating one large enough to
accomodate working room and the angle of repose of the earch;
disposing of the extra earth.

I understand that. Excavating will be done by a pro.

I could be wrong, but you shouldn't have any problems with collapsing the
existing foundation. Once the subfloor is in, which yours has been for ~50
years, an effective box is created. That's why it's normally advised to not
backfill until the subfloor is installed. Just don't hit the wall with the
hoe.