thinking about building a basement

3,901 Views | 17 Replies | Last: 7 yr ago by 62strat
cowtown ag02
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AG

I am in the planning stages of building a house. The lot I have has a fairly steep slope down to a river which provides for a nice view. I was contemplating adding a walk-out basement to the house plan but I am new to the concept of a basement in Texas.

This is a probably a dumb question but I will ask anyways. If the basement will be about 25% to 33% of the size of the main floor of the house and will only be under for example the west side of the house, does the whole main floor have to be sitting on top of the basement foundation. Stated another way, do I have to excavate the entire size of the main floor of the house to build the basement or do I just excavate the portion where the basement will be located and then the rest of the main floor of the house can be on slab or pier and beam.

I would also appreciate any advice others have experience they have with a basement like construction tips, cost to build as comparison with normal building costs, etc. Thanks in advance.
Corps_Ag12
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Location?

Most builders don't like them, but this guy specializes in it.

North Texas Basements

Depending on the foundation conditions, it could get real expensive real quick if you hit rock while excavating.
TexAg1987
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Your basement will only be the size you want it to be. You may have to put the rest of the house on piers, however. An engineer will need to design the foundation, especially on a hill.
cowtown ag02
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The property is southwest of Fort Worth. The soil is sand.. There are no rocks and no issues with excavating.

Why would I need a pier foundation? Not that I am against it just trying to understand the logic.
TexAg1987
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cowtown ag02 said:

The property is southwest of Fort Worth. The soil is sand.. There are no rocks and no issues with excavating.

Why would I need a pier foundation? Not that I am against it just trying to understand the logic.
Usually you strive to have the foundation bearing on the same strata of soil. Digging down deep for a basement then having the rest of the house rest on grade is asking for differential movement. An engineer can test to see if it is a significant difference. It may not be if it is all sand.
cowtown ag02
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1987- thanks for the reply. That is helpful to know. The soil is all sand so we will see when we cross that bridge regarding the soil movement.
62strat
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Basements in Colorado often have crawl spaces. Is that what you mean? Foot print of house is say 1300 sf. Excavated portion of basement could be 750 sf with a 550 sf crawl space. But basements here are walls that are formed and poured in place sitting on continuous footings. Slabs are either floating (slab on grade) or fixed (crawl space under basement floor.)

Honestly you'd be foolish not to excavate the whole thing. It is only incrementally more in cost but you get valuable square footage.
Kenneth_2003
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I'd be inclined to agree. The complication of a split level slab for a structure no bigger than a single family home will probably lead to more cost than the cost of the additional excavation for a full basement.
ToddyHill
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We did the same thing your proposing with our current home in East Tennessee. Our lot slopes from the road down towards a rather large 'creek' that flows into the Tennessee River. The soil is not as dynamic as Texas soils, though clay is definitely predominant.

You may find the slope of the land could benefit you regarding the cost of the basement. In our case, the concrete portion was limited to two sides, which saved a lot of the cost of the labor and raw materials. We chose to finish out the portion of the basement that 'walked-out' to the back, while keeping the other half for storage, a tornado room, etc.

One suggestion...if you do opt for a basement, make sure the exterior walls are sealed and crushed rock and drains are installed.
cowtown ag02
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Thanks everyone. Good input. The main footprint of the house will be around 3,500-4,000 sq ft with the basement probably being around 800-1,000 sq ft.

Excavating will not be a problem but it just seems to me more expensive to dig out that much more ground and then have to build more concrete walls, waterproofing, drainage systems, etc. then to just have to dig out to only fit the basement. The basement will have 3 concrete walls and then one exterior wall will be exposed due to the slope and the walk-out feature.

I understand the concern with the split level foundation however so I will have to address as we proceed but it just seemed in the planning stages that it would be cheaper and easier to only have to excavate for the basement portion only and not the entire footprint of the main floor of the home.
62strat
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Well that is much bigger than I was assuming. I was referring more to homes that are built around here, and a home builder will charge around $15k to have the entire basement excavated, so they get like 1300 instead of 750 (usually two story houses, so footprint is not that big)

$15k for 550sf is $27/sf which is a no brainer, and I'm surprised at how many buyers don't pay it.

1000 compared to 4000 sf is quite a difference I'm sure, and if you do excavate it that is a lot of space to build out and finish, and subsequently heat or cool and light.

I'd still get a quote to excavate and see what you're working with, instead of just assuming it's too much. The earthworks is already out there, 3000sf at ~11 ft is a bit over 1,000cy.. It may not be as much as you think, and having a continuous footing around the house is easy on engineering and building, compared to doing split level stuff. Again, all those trades are already out there doing the basement portion you decide to dig (digging, forming, pouring, engineering), so it's all just incrementally more.
cowtown ag02
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62- that makes sense in your example to excavate the additional space and make a larger living area. In my example, I just don't know how the costs would shake out on excavating the entire footprint of the main house to only have a smaller basement.

I guess my question is if you have a partial basement (which from additional research online I think it is what the basement is called that does not cover the entire first floor footprint of the house), is what type of foundation can be used on the additional main floor of the house? The one poster above indicated that it would probably have to be pier & beam with a crawl space. Could a slab on grade be used for the other portions of the house or is that just too complicated because I assume the portion over the top of the basement has to be wood floor joist and making the tie-in with the concrete slab would be awkward plus the issues mentioned above with differential movement.

Thanks again for all the input, very helpful in planning.
62strat
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cowtown ag02 said:

62- that makes sense in your example to excavate the additional space and make a larger living area. In my example, I just don't know how the costs would shake out on excavating the entire footprint of the main house to only have a smaller basement.

I guess my question is if you have a partial basement (which from additional research online I think it is what the basement is called that does not cover the entire first floor footprint of the house), is what type of foundation can be used on the additional main floor of the house? The one poster above indicated that it would probably have to be pier & beam with a crawl space. Could a slab on grade be used for the other portions of the house or is that just too complicated because I assume the portion over the top of the basement has to be wood floor joist and making the tie-in with the concrete slab would be awkward plus the issues mentioned above with differential movement.

Thanks again for all the input, very helpful in planning.
yeh for a partial basement (again, in CO), the 4 basement concrete walls are on continuous footings (even your 'walk out' has a 4th concrete wall, it just doesn't come above grade. It's 2' tall). Behind your back wall, the crawl space begins, and house above is on pier and beam.

You say "excavate the entire footprint of the main house to only have a smaller basement." that doesn't really make sense.

If you excavated all 4k sf, you might choose to only 'finish' 1000 sf., is that what you mean? Your remaining 3000sf is still there, and it is concrete floor, concrete walls, and exposed beams above. It is simply unfinished. It's not like it's just a dirt hole. It is how every basement starts. You could decide to finish more square footage at a later date. This is the beauty of excavating it to begin with. It costs roughly $30/sf to 'finish' it into basic livable space. With a crawl space, it is a major undertaking to convert it into livable space, likely way more than the initial cost to excavate it and include it in the basement.

It is very common to leave a portion of a fully excavated basement unfinished here as well. You put the furnace/water heater in there, and generally just use it for storage. A ranch home with a large footprint (say, 2500sf), will generally have 1700-2000sf of finished basement, and 500-700 of it remain unfinished.
YellAg2004
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Kenneth_2003
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One other possibility would be a balance of cut and fill across the entire foot print. You'd end up with stairs going up in front and down in the back along with a retaining wall.
cowtown ag02
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62- Thanks for the thoughts. In looking online, it appears that partial basement has two different types of meaning and how it is used. It can be used as you describe to depict a fully excavated basement that has a concrete floor throughout but only a certain portion is finished while the other areas remain "unfinished" and are normally used in their unfinished state for storage or for the mechanical equipment. The other use of the term partial basement is a basement that does not cover the entire footprint of the house such that the remaining area is on a pier and beam with crawlspace.

I was thinking with a partial basement that the contractor would just frame up the 4 walls for the basement and waterproof those walls and then the rest of the main floor of the house will be on pier and beams with a dirt crawlspace. I understand your point that if I ever want to expand it is is much easier if the basement is "unfinished" versus having to dig out the crawlspace. I just don't know what the cost difference would be between doing a pier and beam foundation on roughly 3,000 sq ft of the house versus 3,000 sq ft of unfinished basement.
Corps_Ag12
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Piers are going to cost the same for either design (roughly).

Less mobilizations to form/reinforce/place concrete for a slab & walls (2 pours) = less cost over breaking it up to pour basement slab, pour walls, tie in raber to top of walls & upper slab, pour upper slab. (3-4 pours)

Less engineering if one large basement slab with 3/4 walls on piers vs the structural element of tying in a elevated slab & top of wall with split level slabs & piers.

More excavation for continuous basement BUT this may be able to be spread across the site for fill & leveling.
tyunelil
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Basement should be relatively smaller than 1/4 of the entire first floor. However, it's up to the owner what size it should be. Additionally, it should not affect the foundation of the house and should be situated strategically.
62strat
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tyunelil said:

Basement should be relatively smaller than 1/4 of the entire first floor.
according to who, or what? Never heard that rule of thumb before.
If it is 1/4 of first floor, that sounds like it would be basically a cellar, unless you're talking a large house like ops, 3-4+ksf ranch.
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