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Question DetailsAsked on 2/25/2017

Cost to add basement and 2 floors 40'x20'x8' to existing structure

I want to add a 40'x20'x8' concrete basement, a 40'x20'x8'' wood structure and siding living area and a 20'x20'x8' wood structure with siding upstairs area, shingled roof.

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2 Answers


I hope you mean as an addition, not building up on an existing one-story house - because I am pretty certain that trying to put a basement under a one-story plus 1-1/2 floors on top will cost you more than just tearing the house down and building new, and probably a good deal more than adding on an addition with basement and full first floor and half-size second floor. Also, if you meant adding these on top of the house, not only do most residential areasnot allow more than 2 stories (and if you built-up you would be two full stories plus a half-sized top story), but also most homes are not framed to support a third story. You can usually build a second story on top of an existing first story with 2x4 at 16" or larger studs - usually not a third story without tearing into the walls and beefing up the support system, which means a lot of reworking of existing ground floor finishes after the walls are basically stripped clean.

Cost depends on soil conditions (especially no significant bedrock in the excavation) and topography and groundwater conditions a bit of course, and on whether any utilities or septic system or well or such are in the way, but assuming your property is large enough and zoning allows this expansion, generally will run from about $100-125/SF (not counting basement square footage) in lowest cost areas (generally deep south and up the center of the midwest), more around $150-200/SF in most of the rest of the non-urban US, around $200 give or take in the more urban areas, and $250-400/SF in the costliest urban area - not including land. Of course these are ballpark numbers, not taking into account specifics of design or chosen architectural materials - but good enough for a very rough first look at whether you can even afford to begin talking to an architect about conceptual design and preliminary cost estimate.

An Architect (your Search the List category) is the one to talk to about this as you are going to need plans and specs to get a building permit and for bidders to bid on and eventually build to. And can also work with you on estimated cost and fitting your budget.

And one of the first issues I would bring up with him is the half-size top story - because for not a lot more (since you will already have full basement and ground level done) you can make that a full second story because it just takes a bit more flooring and the added second story walls - roof would already be paid for regardless of which level it sits on, and actually simpler to do as a full roof than a split-level. I would guess an additional $10,000-30,000 (depending on area) would probably get you the rest of the second story in economy finish, which certainly would increase the resale value whether as a separate master bedroom or studio say, or otherwise configured as you wish. Or even with the entire upper floor as an open loft, though with that large an open area that can get into some nasty ventilation drafts if you have cold winters or hot summers.

Answered 3 years ago by LCD



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Answered 3 years ago by Member Services

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