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Question DetailsAsked on 5/19/2016

Cost to lay foundation under existing 1500 square foot guest house

When the guest house was built there was concrete poured around the perimeter of the house but the inner part is all dirt. I would like it filled with concrete to have a complete slab under the house.

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1 Answer


For a foundation, supporting the whole house - probably in the range of $15,000-30,000 because it means jacking the house up to do it - a total foundation replacement.

If you mean just concreting in the crawlspace, assuming that will still leave you at least 8 inches or so clearance (for moisture and insect protection - height varies by locale and can be as high as 18 inches) if no utility runs under there, or more like 18-24 inches minimum if there are pipes and ducts and wiring that might need repair, then probably about $7,500-10,000 range for a "clean" slab - properly graded and smooth like for a basement, or as cheap as maybe $1000-2000 for 1-1/2 to 2 inches of shotcrete ("shot" or blown in, like many swimming pools are done) quite spiky rough concrete, or about twice that amount if rough floated but not troweled so still pretty rough but "flat" - suitable for crawling around in without special protection from the coarse as-shot product.

You don't say WHY you want this - is your foundation settling (in which case without fixing the water or soil condition causing it might not help any), or what. If trying to stop moisture issues from the ground concrete slab will not help much - a LOT of water vapor can pass through concrete that is moist on the bottom - on the order of probably up to 10-20 gallons/day for that size slab. Generally, a vapor barrier is put down first and sealed tight to the foundation wall to alleviate this issue - and maybe underdrains if the water level gets up to the existing crawlspace surface.

Be sure, if considering this, to weigh the benefit to you and any potential resale benefit (if any) versus cost - because foundation redo's in particular can commonly get into a situation where it is cheaper to sell and move to a house that suits your needs as-is, though I do realize that this being a guest house situation that probably does not apply here - but don't overspend on a building that is not worth much inthe first place.

Perhaps if you reply back explaining WHY you want to do this and what issue you are trying to solve or end result you are shooting for, using the yellow Answer This Question button right below your question, I can help with suggested solution(s) more keyed to your specific situation.

Answered 3 years ago by LCD

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