Those aren't foundations as they drop down just like any normal wall when cutaway and walls down are selected. I can build a door that leads inside of the staircase (Which is what I'm trying to do I just need a half wall to back the sink).
Build a wall. Next to it dig down four clicks and build a parallel wall. Then CTRL+SHIFT+C and type in constrainFloorElevation false. Select the level terrain tool and place it on top of the lower wall. Drag from that wall to the higher wall and notice it cut itself down.