I'm wanting to adding a addition with a basement next to an existing home that sits on a crawl space. The exisiting crawls space it about 5' hight so the new basement will need to drop down about 4' below the existing footings. I've had contractors propose 2 ways. 1. Remove the common exisiting wall and footing, drop it down 4 feet and create a new footing and wall. 2. Create a new wall and footing outside the exisiting footing. I've been told that the second way could compromise the exisiting footing. Is there a recommended solution? Thanks

This message has been edited. Last edited by: NCSam,

The right way to do it is to underpin the existing footing. The wrongest way to do it is to build a new wall next to the old one. In between is the remove and replace method. A lot depends on soil types, the loads on that particular wall, etc.

Chances are when you try to remove and replace it, you'll lose soil from under your crawl space floor, and probably the floor will crack. You also risk compromising the soil under the footings that meet the removed wall. Underpinning is a lot easier and it is much safer.

I would agree with richard, you could lead your self into some trouble with the 2 ways you listed.Underpinning is a much better way to have your addition done.

Thanks for your suggestions. Now, what is underpinning and shouldn't my contractor know about this? Do I need and enginner to look at it? Thanks

Underpinning is basically extending your existing footings downward a little width at a time, using concrete.

Basically, what you do is dig out 2 or 3 foot wide sections under the footing, to the depth of your new basement or somewhat below, leaving 4 to 6 feet of earth between digs. Then you put up a wood form and fill the dig with concrete. After a couple of days, you dig more 2 or 3 foot sections, again spacing them 4 to 6 feet apart, and form and fill those with concrete. Repeat this process until the existing wall is fully supported by the new concrete.

Thanks that explains it. Seems the web mostly recommends it as a repair. Would any excavator know how to do this or does it take special company? As you can see I don't know much about this but I figure the foundation is the most important part to get done right.

Foundation construction isn't usually the province of excavators. An experienced general contractor ought to know about the technique, as should experienced masonry or concrete contractors. It is a pretty standard practice.