Expert: Richard Taylor, AIA - 4/17/2009

Question
   Hi, I am going to be putting an addition onto my house, it has an existing concrete foundation/basement (its in rough shape and barely 5' 6" down there. I will be putting on a 24x20ft addition with two stories and a full basement.
   My question is, how do I attach the new foundation/basement to the existing foundation/basement?

Answer
Hi Rob - thanks for your question.

If I'm reading you right, you want to put a deeper basement next to a shallower one...there are three ways.  All address the different footing heights differently.  You have to be careful in each case to NOT undercut the existing footings without proper shoring and support.

First is building the new foundation at the same height as the old, then gradually stepping it down to the new lower foundation.  This is the least disruptive of the existing foundation, but will reduce the size of the new basement significantly.

Second is pouring a "bulkhead" next to the exposed side of the old foundation, thick enough and reinforced enough to resist the horizontal thrust of the old foundation (requires engineering).  This solution will take some space from the new basement to accommodate the thickness of the bulkhead.

Third is undercutting the existing footing and pouring a new footing and foundation wall below it to the new level.  This one also requires some engineering and is the trickiest to build - but doesn't take up any space in the new basement.

Hope this helps!

Richard Taylor, AIA
www.rtastudio.com
www.rtaplans.com