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Question DetailsAsked on 3/6/2017

How much/ what steps needed to add a 850 sqft basement underneath an existing home with Crawl space.

I've been told a basement can be poured just inside/around the existing foundation. Not looking to lift the house. How much work could I do myself, and at point/when should I involve a professional beyond advice?

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Pouring a new foundation AROUND the outside of your existing one is a repair technique, mnost commonly used for deteriorating brick or fieldstone foundations to stabilize them - basically casting a shell around the existing wall (sometimes on both sides). Does NOT do anything for your case, because the new foundation is OUTSIDE the existing one but by itself cannot (without a lot of special engineering and construction) support the house because it is outside the house walls, so cannot take out the existing foundation to build the basement.

For your case, you can (difficulty depending on how well the existing soil supports the base of the existing foundation and on whether it is damp or wet) put a new foundation further down either directly under the existing foundation, or tied to the inside of the existing footing (which then becomes a ledger or "half wall" in the finished basement. Usually done incrementally alternating in sections, leaving existing soil in place for a section then excavate out for a section to install the new foundation - pouring all those segments in one pass, then going back and taking out all the remaining alternate soil sections and pouring new footings and wall sections there.

On DIY jobs, because doing it bit by bit is feasible and economic if you don't value your time, sometimes the work is just done 3-4 feet at a time, working around the foundation segment by segment rather than in alternating segments. Google on the web about DIY basements under existing houses - LOTS of blogs about it - and a LOT of people have given up shortly after starting. LOT of manual work (mostly with poor headroom), and requires doing the rebar in short segments or a lot of rebar bending out of the way then bending back into place (if allowed by your code officials - some will not allow that so you have to use expensive rebar connectors by the hundreds, if allowed in your area).

Plus DIY jobs commonly have to pay additional inspection callout fees if doing it incrementally, if the inspector wants to see each section of rebar and forming before pouring rather than maybe just the first one to see you know what you are doing.

Either way, you would need an evaluation and design by the Structural Engineer (the Search the List category) to get a building permit and soils evaluation by a Geotechnical Engineer, to tell you how much existing foundation support (under and/or down along inside edge) you can remove at a time, and what sort of temporary support you need to put in to support the excavated-out sections as you work. Depends a LOT on soil and water conditions - sometimes you can get away with 3-5 feet of unsupported footer hanging in mid-air for a few days while excavating out the fill at that area and putting in rebar and concrete or concrete/block - in other cases temporary support beams have to run from excavations and bearing pad outside the footer directly under (and supporting the footer) to a bearing pad on the inside of the basement, located every 2-3 feet along the footer untill supported by the new concrete/ concrete footer and block.

Generally, you cannot just dig down along the inside face of the existing footer the 2-6 feet needed to get the 8 inches or so below finish basement subgrade level and then pour a new footer and wall to tie into the existing one, because the existing one is bearing on the soil under the footer, and is using that soil below its elevation but inside the basement footprint for support, generally extending out from the footer at an angle of 1V:1H to 1V:3H (vertical to horizontal) depending on soil type and water conditions, so without removing that wedge of supporting soil you would lose a great deal of basement space - hence the need for temporary supports while taking out the supporting soil to put in the new vertical foundation.

And of course, if a DIY job you are talking significant effort hand making concrete (or having several deliveries at different times and getting the concrete to where you need it - or paying for pumping and a fair amount of standby time - for small quantity segmental footings. And then a LOT of effort getting the dirt out.

Couple of hints on DIY'ing to discuss with engineer -

1) possibly digging out niches under the footer upfront, then using precast adjustable supporting piers or disposable steel screw jacks under the existing wall to support it in longish sections (like 6-12 feet per segment), then placing rebar for that entire section and pouring longer sections of wall than 3-5 feet at one time, incorpoprating the piers or jacks in the wall - or with the piers being intermediate support columns (with blockout holes for the longitudinal rebar to pass through) then pouring connecting segments of wall between the columns. And you can do maybe one section like this per each wall all at one time, so you can do the entire concreting job (after the piers) in only 2 concrete pours delivered by truck - though of course you have more forming to do and have in place that way (rental forms probably).

2) not forming the "outside" of the new wall - using the existing soil as forming on the outside. Works only in cases where you do not need to waterproof the new foundation and the existing soil will stand basically vertically for days.

3) in very stiff and good bearing condition soils like compact tills or clays or rock, it is sometimes possible to dig straight down from or even in under the inside edge of the existing footer and pour (in shortish segments) new footer at desired new basement depth (below new slab elevation) and a vertical panel wall up to the front face or under the front edge of the existing footer (which will support the outside soil), doweling the rebar into the existing footer, mostly or totally eliminating the need for temporary footer support. Makes for a 4-8" half-wall (the existing footer) but that is common in daylight basements (where bottom half if foundation and top half is studwall) so does not look so odd, especially if you build shelving or such on the halfwall portion.

Here are links to a number of previous similar and related questions (not every detail matches your case, but combined they pretty much cover the subject):

Answered 3 years ago by LCD

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