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Building an addition, full basement or crawl space?

Building an addition, full basement or crawl space?

Old 10-29-03, 01:46 PM
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Building an addition, full basement or crawl space?


I'm still a year out from building an addition to my house, it will be approx. 12x30 along the side of the house & will have HVAC. At this point I'm mostly dreaming - how should I do this or that. At some point I'll be hiring an excavator to do the work, but the more information I have now the better I can understand the whole process.

The original structure of the house has a full basement. For the addition I could either:
1) Pour a footed slab;
2) Dig out a crawl space (concrete) & build on top
3) Do the full basement routine & expand the existing basement.

I'd appreciate any input here, again there's now final decision but I'd like to start thinking of the pros & cons of different approaches.

One of the things I'm interested in getting a sense of is cost of a slab vs. a crawl space vs. a full basement. I know costs are going to vary quite a bit do to location, soil type, etc. but any rough estimates would be appreciated. If that is hard to do, would anyone have any idea of the relative price increase of these different options? For example, a basement is roughly 3x the cost (or whatever) of a crawl space. That will help in the planning phase.

One of the things I'm thinking about is if the cost isn't too much more, I would probably favor a full basement. I would imagine that the cost of that basement per sq. ft. would be significantly lower than an above grade addition, there may be some added value there.

Thanks in advance!
Old 10-29-03, 09:22 PM
Join Date: Jan 2002
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How high is your existing house over the grade? I never liked slabs that much, I'll take a crawlspace over a slab anyday, especially if the existing house is over 16" of grade.
The difficult part is not really in excavating and pouring the basement walls (you'll use about 4 times more concrete + floor with a basement as opposed to a crawlspace), but connecting the new basement to the old one. Cutting a door into a 8" thick steel reinforced concrete wall is no easy task.
Old 10-30-03, 04:22 AM
Join Date: Oct 2001
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Years ago, my firm used to do residential (we now only do commercial work) and we ran across this question occaisionally. Every time we did the analysis for the client, they went with the basement option. BTW, like trinitro, we always liked the crawl space vs. slab on grade.

Assuming you need a wall down to a frost footing, then the costs are not that much more. All you're buying extra is a thicker wall, an extra few feet of depth for the wall and excavation (depending on code required frost depth), and a little bit for heating and electrical cost for the basement space. Since your dirt, masonry and concrete contractors are on site anyway, you've already paid for their mobilization and demobilization costs. That basement space becomes quite cheap space per square foot.

It's been too long since we did that kind of design, so costs we looked at then are long outdated. I'm sure someone on this forum can help with some relative costs.


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