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Question DetailsAsked on 4/5/2016

Can i add a basement under a home with a concrete slab?

The only way to update the water/electronic of the house we have to tear up the slab. If were already tearing it up. Then cant i add a basement

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I would first question having to tear the slab out to replace the electrical and plumbing. If your contractor is saying you have to tear up much of the slab I would be looking at talking to different contractors who have done this before. About the only time you have to demolish major portions of a slab on grade is if you are having soils problems causing settlement and breakage, or if you are replacing an in-slab heating loop system, which is very rare as a repair.


Obviously the sewer pipes (assuming you have gravity flow to the septic tank or street) have to be below the lowest floor with water use to avoid using a sewage lift pump - presumably your first floor, so yes you have to cut a narrow (typically about 12 inch wide) strip or several holes a foot or two in diameter (which are better to preserve some slab integrity - though any cuts over about 1' in maximum dimension should have the rebar that was cut replaced with doweled-in rebar during the repair) through the slab to replace the sewer pipes where they pass under the slab. In some cases there is an alternative shorter routing which reduces the need for a lot of that, depending on the elevations of slab and sewer pipe outside the house. However, you are not taking anywhere near the entire slab out - normally not even 10% of surface area.


However, water and gas and electrical generally you can abandon anything under the slab and run the new plumbing through the walls or floor joists - commonly cheaper assuming you have ordinary plaster or drywall walls and wood stud framing (not brick or concrete or stone construction), though does involve some destruction to your interior finishes. In cold areas the water commonly comes in underground to inside the house, then into ceiling or walls from there, to minimize freezing risk - though you can accomplish the same thing with an insulated bumpout or utilidor on the outside of the house.

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But to directly answer your question of doing a basement IF you were taking the slab out - not at all easily. For one thing, the house is fastened directly to the slab at all walls, which sit directly on the slab, so there is no floor framing that you can use to lift the house up to remove the slab. In fact, you could not reasonably remove the aslab anyway because you would be removing all the support under the walls.


Also, the slab is not designed to free-span over a basement, so even if you did insist on putting in a basement you would have to put in very substantial steel beams as floor joists to support the slab, jack them (and entire house) up to dig the foundation and put in foundation walls, then lower the house onto the new foundation. Because all this would be underground work, generally not at all economic and you can commonly (assuming you have the space) build a comparable sized adjacent addition with foundation and one floor or a two-story addition on slab-on-grade as cheap or cheaper than retrofitting a basement under a slab house.


For more details and rough cost estimates for your case, an Architect would be the person to turn to, but I would give about 5:1 odds it would not be economic - commonly in the $40-60,000 range (for unfinished basement) depending on site conditions and house design, so commonly into the $100,000 or more range finished. Generally far more than moving a normal frame house on footers or piers to a new site or foundation.


Here is a prior similar question with response in more detail on the issues of doing what you are talking about FYI -


https://answers.angieslist.com/can-ba...

Answered 4 years ago by LCD




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