HI, I would like to know if anyone has ever added a partial basement to an existing slab-on-grade house by sawing an opening in the thin part of the slab, removing soil underneath, and installing the floor slab and block walls. The house in question is on dry, sandy loam, well above any water table, and in the southern US (high frost line). Space would be used for utilities and storage, as well as storm protection.
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I've seen essentially the same thing done here on old houses with crawlspaces. They excavate and build deeper foundation walls that are inset from the original walls.
It strikes me as slow and risky. One small section is excavated at a time.
I think it would be a lot easier and cheaper to add on to the house and put the basement only under the addition.
This topic comes up regularly at Breaktime. You might want to post the question there as well.