We are considering buying a lot on which abt 2400 square fairly rectangle walk out basement foundation has been already built. The lot was flat to begin with, but the builder has created a walk out option by making a slope of dirt on the front and sides slope back to a flat backyard. It has retaining walls on the sides. We want to build an icf one and a half story home over the walk out basement. Do we have to use the same foundation if we want to build on it or can we change the plans without tearing it up? Do you see any negatives of using such a type of walk out? Thanks so much.

Do you have the plans that the previous owner intended to build?  Without knowing the original design I would be hesitant to use any interior columns or bearing walls.  You will likely need the services of an engineer. 

If permit plans were filed, you may be able to get a copy from your local building department or perhaps the contractor who did the work (if you know).

Do you know if there are perimeter foundation drains?

Bruce

Yes there are perimeter foundation drains. They are willing to give us plans but we don't like the floor plan. thanks for the reply

You had better hope they put rebar in the foundation. If you are confident the foundation is solid and only plan on putting ICF walls directly on top of the old walls, you can use it. If you wanted any insets or jutouts that aren't in the block wall, you would have to switch to stick framing for that small part and possibly cantilever over the foundation. Also, you will have to insulate the basement walls and floors if you want that to be heated living space.

If this was my house, I would tear it down and install a foundation that I would KNOW would support the added weight of ICF walls, and not have to switch systems for jutouts.

Thanks