I am adding a 14 X 28 addition to my house. The existing house has a 4 Ft. concrete block foundation on top of a 10 x 16 in. footing.The new addition is to have 8 ft. concrete wall on three new sides. I want to extend the existing block wall to close off the basement. What should I do? I don,t want to undermine the existing foundation.
A couple of opitions that I have considered are: 1. Pour a new footing and 4 ft. concete or block wall inside the additin. The problem with this is loss of space in the addition. Approx. 12 in. 8 in. wall plus side of old footing.

2. Undermine the existing footing using temporary support , pour a new footing with a block wall up to underside of old footing. The problem with this option is trying to get blocks or concrete under old footing.And if I pour a new wall I will have to leave one side of the forms buried under existing footing.

What should I do?