I built a home in 2000; basements are not the norm in my area and, although my site is sandy, well-drained, etc., my builder did not know basements so I don’t have one and I really regret it. I am thinking of adding a room underneath my den which is about 18 x 22, either by excavating underneath the slab or by sawing most of the floor out then excavating, pouring a concrete floor and walling it with foam-formed concrete, then repouring the floor over steel decking. The stairs are on one end of this space, so the “down” stars can be added underneath, where they would have been anyway. What do you think?
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<<my site is sandy,>>
As long as you figure out a way to contain the soil that is already under the house (stabilize it against a displacement failure) and support the existing foundation during the excavation, there is no reason why you couldn't do this.
But if you don't use some kind of shoring you could be setting yourself up for some serious trouble.
I also think it might be cheaper to add a basement next to the house and put an addition over top of that.
My house is on a raised slab, about 18 inches or so, with sandy fill hauled in. the original soil is a thick layer of sandy loam over more firm material: it generally will support a dig with vertical walls quite well. I do agree in general with your recommendations though. Generally what I propose would be well away from the exterior of the house, therefore the footings: at least 10-12 feet or more inside the footings which were dug well into undisturbed soil (our frost line here in northern Louisiana is practically non-existant, though). I just thought it would be a good what if to discuss. I am an engineer but homebuilding is far from my speciality; I do think up some good 'uns sometimes, though.
Sounds like you've got a handle on it.If you ever decide to really do this please post the story and photos, it would be interesting.
I remember in L.A. some builders were adding basements under beautifull homes near downtown. The owners were tired of parking thier cars on the street.
So they would take a small loader and start digging a ramp down to and under the house with shoring.
After you were done you could drive up to an electric gate, open it, and park under your house. All secure and up to modern standards.
Plus, you lived in an cool old house near work.