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Converting a crawlspace into a basement

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  • Converting a crawlspace into a basement

    My client wants to convert his crawl space into a basement without raising the building. The crawlspace is 24x40 with a girder down the center line. Where can I find info on the procedure and solutions for this project? Thanks for any direction you can supply!

  • #2
    Re: Converting a crawlspace into a basement

    Your client should be made aware of the fact that it's one Hell of a lot cheaper to add on the house, if possible, than to go under it.


    • #3
      Re: Converting a crawlspace into a basement


      The "procedure and solutions" aren't going to be written in stone anywhere, that's for sure. But basically, you'll be doing a lot of hand digging with cheap labor, and a whole lot of underpinning and shoring. As Dick said, it'd be a WHOLE lot cheaper to add on. If I had to wager a quick guess at the costs to go from a 2' crawlspace to an 8' basement, I'd say it would be:

      -Excavation- 310 CY by hand @ $120/CY = $37,200
      -Underpin foundation walls- 1,024 SF @ $35/SF = $35,840
      - New footings- 128 LF @ 30/LF = $3840
      - New spread footings 3 @ $300 = $900
      - Temp shoring and bracing- minimum $10,000
      - 4" Slab- 960 SF @ $4.75/SF = $4560

      That's $92,370 cost. Add your OH&P and it's an easy $140-150K. That's about $140/SF for raw, subterranean space with no finishes, no mechanicals, etc. Not worth it in my book, but I'm not the customer.



      • #4
        Re: Converting a crawlspace into a basement

        I talked with a guy in Colorado a couple of years ago who had a business adding basements under existing houses. His technique was to use a Bobcat to dig a ramp to give access under the house, then use it to excate the area. After shoring and excavation, he built a concrete block wall under the perimeter beam. Never saw it done but he said he made decent money on the project.

        I checked into adding space under my own house using a different approach. My plan was to hire a house moving company to raise my house and put it on cribs/shoring outside the perimeter of the house foundation. Obviously, when you're raising the structure you have to do some temporary plumbing modifications. Next comes mechanical excavation, placing the foundation, and lowering the house back down. The moving company would charge around $10K and was offset by the relatively low cost of mechanical excavation. The total estimated budget was around $50K cost at that time or about $50/sf, substantially cheaper than adding above ground space. Looking at it one way, it's just like building a typical basement with the added costs of structure lifting and all that is implied by doing this.

        The technology for doing this isn't that complicated. I chose not to do it at my place because the property values didn't support additional investment.


        • #5
          Re: Converting a crawlspace into a basement

          Did just this 18 mos ago for a client. House movers raised the unit and blocked it up. Dug it out with a small skid loader, and poured foundation. They set it back down. The lift was about $10K. Bobcat work took 2 days. Then, it was just a normal pour... Not as expensive as I suspected. You need a good TRUSTWORTHY person on the bobcat, as they can't just go crazy aroung those pilings. Added benefit, was that we were working in the shade throughout July, as opposed to in the sun...

          Remember, there's some added cost in getting rid of the dirt/rubble of the dig. Our customer used it to fill in a washout only 100' away, so that was his problem, otherwise, we'd have had to charge to haul it away. There was lots of old stone, cast iron piping, etc. mixed in, so it might be an issue some places.

          Overall cost was pretty decent, I felt. We made out OK, and the customer saved money on taxes, as the county didn't really have option to raise them, as the footprint of the house never changed. Just pulled permit as "foundation repair"...not sure it was entirely kosher, but no one checked. Besides, in tighter quarters (not our issue) you can't always add on...


          • #6
            Re: Converting a crawlspace into a basement


            What was the cost per square foot compared to an above ground addition?


            • #7
              Re: Converting a crawlspace into a basement

              Hmmm...not sure I can calculate that easily as we added on 8x16 room (to square up the house) plus 10'x35' covered porch...never actually considered just "adding on"...

              What I can tell you is that it was logically (in my mind, anyway) cheaper, as they gained approx. 1000 sq ft of living space for just the cost of the lift and concrete. The entire project was about $32. That includes finishing out the basement living areas (bedroom, family room, no bathroom, although a space is there w/subgrade plumbing in place to allow for it in the future). So I guess you could call it $32/sq ft., although part of that cost was for structural steel for porch. Not sure if that answers the Q or not...