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Old 03-11-2009, 12:01 PM   #1
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Footing/Foundation for Addition

I have a fair amount of home renovation experience, but not so much with masonry/foundation work. I've decided to take on the task of building a small three story addition on my DC rowhouse. I'm including pictures of the site for reference.

Basically, the addition will enclose the dog ear or set back (according to the DC zoning office it is officially a "court") on all three levels. On the right hand side and the back side, the addition will simply tie into the existing structure using ledger boards on the left, the majority of the addition will be built on top of the existing party wall (no worries, have cleared this with zoning, building inspector and neighbor), although as you can see, the left side extends only 7.5' from the back wall while the right side extends 10' (and the addition will come out to that point). The basement floor of the addition will be concrete slab, although we'd like to raise it up from where it is currently about 3" to match the existing interior floor level.

With all that as background, I am trying to figure out the best option for a footer/foundation in this application. Per zoning regulation, the footer must be at least 36" below grade and at least 18" wide. Also, the foundation must extend at least 6" above grade. I'd like to come up with a solution that would allow me to pour the footer, foundation and slab in one monolithic pour. Is that even remotely possible, a slab on grade with turned down footer?

I'd like to come up with the simplest yet most effective solution. Given how small it is (the footer/foundation will extend out from the left wall about 2.5' and then over 5', the slab is just about 4.5'x10') I'd like to minimize the number of concrete pours I have to have. I'd also like to do as much of it myself as possible.

Any thoughts?
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Old 03-11-2009, 01:43 PM   #2
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Nothing requires the footing and the foundation to be two different elements. Monolithic grade beam type footings are common. Yours would be 18" wide by however high you need it to be (4' more or less). They take a considerable amount of concrete.

Have you had your plans reviewed by the city yet? They'll likely require an engineer to design the footings for you. I'll also be very curious to hear what they have to say about the fire separation issues that will inevitably be encountered (as we mentioned in a previous thread of yours).
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