Abstract: A system of stem wall foundation blocks is a set of reinforced precast, post-tensioned modular footing and stem wall foundation blocks with a unique interconnecting design optimized for both residential and commercial applications that eliminates the need for on-site manufacturing of a foundation mold. The apparatus provides a unique modular foundation design for a variety of applications within construction. A series of wall blocks of different shapes and arrangements interlock, thus enabling users to modify the dimensions of their foundation mold to a desired purpose. Engineered bolts and lapped connections enable the modular wall pieces to securely connect after they have interlocked. This arrangement enables construction workers to move the interlocking blocks into position and then begin pouring concrete inside to lay the foundation.