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Converting crawlspace additions into full basement rooms

Converting crawlspace additions into full basement rooms

Old 01-01-08, 07:14 PM
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Converting crawlspace additions into full basement rooms

My home was built somewhere around the 1920's and has a full basement. In the 1970's the previous owners added on an addition of 18x7 in the front of the house and an addition of 14x20 in the back. I dug down outside to see if it would just be a matter of excavating dirt... but the foundation only goes down 30". My plan is to cut doorways in the foundation walls excavate the dirt to the level of my current basement and somehow reinforce the foundation from underneath. any ideas of what it would take to extend the foundation downward? I really have no idea what it would cost to hire a proffessional, but i would definately consider it. Any help is greatly appreciated
Old 01-01-08, 08:45 PM
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You definitely need a professional and this is going to cost a lot more than you think.
Old 01-02-08, 01:21 PM
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Digging basements out under existing homes is fairly common around here, usually to about 24" below the original basement floor grade for additional headroom. The major concern is to not remove too much fill from the inside of the foundation at one time so as to avoid pressure from existing fill on the outside pushing the foundation/foundation wall in and collapsing. Conditions impact the decision, but the process (called "underpinning") is usually done one corner at a time with wall section lengths of about 10'-15'. That length can vary quite a bit with conditions and the daring of the contractor. I've seen recommendations of only 4' also. It might be easier in your situation to temporarily support the addition and just remove existing block etc., dig new hole., build new foundation wall, maybe not.

The cost here to dig out 2'-0" of dirt from an approx. 12' x 40' basement, pour foundation and slab runs about $15-20,000 with a contractor. You have a lot more digging to do, it might make sense to take out a wall and get a Bobcat in there.

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