Cost of extending below-grade basement


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Old 10-20-16, 09:23 AM
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Cost of extending below-grade basement

I realize this is likely a difficult question to answer, but I'm trying to get a ballpark estimate of how much it would cost to remove a 10 ft x 8 ft section of the concrete foundation wall in my basement to extend the living space by 250 sq ft and make the basement a walkout. I'll be having a foundation poured for an addition above, so it would be a matter of excavating more dirt, removing the existing wall to make that area livable space, installing a door, and presumably installing a 12 ft beam above where the existing wall was to support the floor above. Would $5,000 be a reasonable estimate for this work? I would be doing all the internal finishes (insulation, HVAC, drywall, electrical), so I'm just looking for the cost of the items I listed. I can post a couple pictures in later today or tomorrow to aid in the discussion.
 
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Old 10-21-16, 06:58 AM
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Pricing

Make detailed listing of the materials needed and price each item at your local builders supply.

An important part of this will be the specifications for the beam spanning the area where the basement wall will be removed. Is the wall above a load bearing wall?

Some photos from the outside would help.
 
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Old 10-21-16, 07:09 AM
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One of the first things to consider is the permitting required. Of course a building permit will be required but they may not allow the work without an Engineer to design the beam and foundation/footers required. Up where you are permits may be a few hundred then the Engineer could be about $1'000 so I'm guessing you're out $1'500+ before you break ground.

Then there is the excavation. Do you have room on your property for the spoil (removed dirt)? If the dirt can stay on your property that could save you a couple hundred per dump truck load. What other excavating will be required for your additon? If a backhoe or track hoe will be on site already that will help with the cost as simply getting the machine on site can be a few hundred.

I'm guessing that the permitting, Engineering, excavation and demolition will eat up your $5'000.
 
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Old 10-21-16, 07:43 AM
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Best process is to get three or more local contractors to bid the job so you can compare and ask questions.

That said, I agree that $5K does not seem sufficient.
 
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Old 10-21-16, 08:17 AM
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An important part of this will be the specifications for the beam spanning the area where the basement wall will be removed. Is the wall above a load bearing wall?
Yes, the wall above on the 1st floor is currently load bearing, but a 10' section of that wall will be removed to extend into the new addition. Therefore, a beam will be installed above it and the load carried down on either side to the foundation below. So I will then need another beam in the basement to carry the load of the 1st floor above.

I do not have room on my property to keep the dirt (it's only 0.1 acre).

Some photos from the outside would help.
Working on it. I will post some later this evening or tomorrow.

BTW, the existing foundation wall is about six feet below grade at the highest point, and four feet at the lowest.
 
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Old 10-21-16, 12:09 PM
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Here are drawings of what I am assuming will need to happen to open up a ten foot section of the load bearing dining room wall on the first floor (roof rafters not shown, but rest on top of dining room wall) and a 10 foot section of the foundation wall in order to make the area underneath the new addition livable space. Is this about right? BTW, the 1st floor floor joists run perpendicular to this wall, and the grayed out portion in the second image is the new foundation wall 9 feet into the page. I have no clue if a steel beam is necessary for the basement support, but that is what I drew.

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Old 10-22-16, 05:18 AM
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If you're tossing out the idea to removing the wall then you should not need the steel beam as you'll have the wall to bear the load. If you were sticking with removing the wall you would need it but now that you're leaving the wall then it's not needed. You will need a header over the egress window but that will be much less expensive than a full length beam.

Since you are going with a beam up on the main floor you'll need to check what the Engineer or inspector want to see below to support the point load where the columns meet the floor. This could continue down through the basement walls to the footer. How are your basement walls constructed? You said concrete but is it solid poured concrete or CMU block? And if it's block do you know if they are filled?
 
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Old 10-22-16, 05:42 AM
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To clarify, the light gray portion of wall in the second image is pushed back nine feet, so the steel beam I am showing is for supporting the dining room floor joists that were originally supported by the old foundation wall (dark gray). It's a little difficult to show perspective using Microsoft Word. Foundation is poured concrete, 12" thick, 4,000 psi.

I'm wondering if removing the foundation wall and excavating underneath for 180 sq ft additional space and natural light may not be worth the cost and effort. I'll post some pictures and a floor plan shortly.
 
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Old 10-22-16, 08:16 AM
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If you do go with the addition and the steel beam then the bearing capacity of your existing basement walls but at 12" thick poured concrete you should be in good shape. The big question for a Engineer is the footer under the cut ends of your old basement wall and whether is sufficient to withstand the point load.
 
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Old 10-22-16, 06:24 PM
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The big question for a Engineer is the footer under the cut ends of your old basement wall and whether is sufficient to withstand the point load.
The area above the dining room is a cathedral ceiling up to the roof, so the only thing supported by that corner would be maybe a five foot wide section of roof and a ten foot span.
 
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Old 10-23-16, 04:47 AM
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A normal basement wall distributes the load evenly over the footer and eventually the ground. Adding the beams takes all the load for that section and concentrates it under your beam ends. Your footers very well may be adequate but it is one issue an inspector will want to verify or may require an Engineer's approval.
 
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Old 10-23-16, 05:15 PM
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Here are the photos I promised. Sorry about the foliage. I need to remove that one of these days in the near future. The dining room addition will come back about three feet from the corner (to the left of the slider) and nine feet over to the left. This is the area I would want excavated and about a 10 ft. section of foundation removed to expand the finished basement below.

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Old 10-24-16, 06:07 AM
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I definetely think adding a basement under the addition is a better idea than trying to make it open to allow light into the basement windows. The big thing will be deciding if the extra cost of basement vs a crawl space is worth the money.

If you are concerned about loosing the light in the basement if you cover the windows you could cut in windows on the end of the house. My big concern there is the odd stairs going to a sliding door. I assume in the future a deck will end up there which would block the light from a relocated window.
 
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Old 10-24-16, 10:03 AM
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If you are concerned about loosing the light in the basement if you cover the windows you could cut in windows on the end of the house. My big concern there is the odd stairs going to a sliding door. I assume in the future a deck will end up there which would block the light from a relocated window.
The stairs would be removed. The previous owner installed them and didn't do a very good job. I'm surprised they've held up for this long. I wasn't sure if cutting new windows into the backside of the foundation was even a possibility. If so, I think I may go the crawlspace route. I'll get estimates for both regardless.
 
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Old 10-26-16, 08:11 AM
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I've decided not to bump out the dining room and the basement. I'll just be doing the garage and eventually installing a deck where I initially planned on putting the dining room. Not only do I not want to over build for the neighborhood, I forgot that I'll be losing half of my front porch, which is where I store my firewood, bicycles, and gas grill. I need some outdoor space and a deck will be perfect. Plus I will save a ton by only doing a single car attached garage. And, I won't be covering those two foundation windows. Granted they'll be under the deck, but I'm okay with that. I can always put a glass floor on the deck
 
 

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