A unitary, composite wall foundation and floor slab structure and system is provided by a pre-stressed cast concrete structure having a wall foundation region, a floor slab region and a transitional region therebetween and strands passing through all of those regions. One or more such structures are also post-tensioned using cables passing through tube embedded in the cast concrete structure and, when a plurality of such structures are employed in a modular manner, such post-tensioning draws all such modules into a larger unitary composite wall foundation and floor slab structure. The transition region may be further reinforced by angular reinforcing elements. Intermediate and end sections of varied shape may be used to complete the structure.