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Question DetailsAsked on 11/17/2014

can a basement be put under a house that sits on a concrete slab

I have a house that currently sits on a concrete slab. My wife would really like to have a basement, is this even possible

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Yes, but not at all easily - here is a prior similar question with responses - I suggest you read carefully, keeping in mind some of their situation is similar to yours but not all parts of the question or responses may apply to your case -


https://answers.angieslist.com/I-addi...


Generally, a slab house just has walls sitting on and fastened to the concrete slab, with no wood framing under it like a house built on a concrete or block strip footer has. Therefore, there are no structural joists under it that support the ground floor and can be used to pick it up off the foundation, deepen the foundation, then set the house back down. Therefore, it would be necessary to lift the concrete slab and house together, support it, put in the basement, lower the slab back down onto a number of strong supports, then cut through the slab to provide the entrance to the basement.

Because the concrete slab is designed to sit on the ground, not to span an open basement, it is nowhere strong enough to do that without a number of heavy beams under it, spanning from foundation wall to wall. All in all, this type of underpinning job can be done and is done in commercial jobs like historic preservation where the building has to be saved, but I would say no way is it going to be economic for you. For a very off-the-cuff number, I would say $100,000 would probably be cutting it short, and that is probably pushing (or exceeding) the cost of building roughly a 800-1000 SF addition to your house. I would just build an addition - even a garage, workshop, or sunroom with a basement under it if you really want a basement - though if it comes down to building an addition or a basement most people would take the above-ground floors over a basement - although a basement is cheap square footage if building over it anyway, barring soils issues working against a basement.


Which brings up one other thing - you did not say what part of the country you live in - but if basements are rare in your area, there may be a very good reason - shallow bedrock, expansive soils, high groundwater (either year-around or during floods), poor soil conditions under the existing or trucked-in surficial soils (like in silty floodplain or peat bog areas where gravel fill is placed over the poor materials as a building pad), etc.

Answered 4 years ago by LCD




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