A unitized building and a method of mass-producing the same that features the use of percast wall and floor components with the wall components being provided with vertical voids that are in registry with the spacing between adjacent horizontal floor sections which are, in turn, supported on said wall sections in spanning relationship thereto. The floor components are precast prior to delivery to the site so that upon in situ pouring of the floor slabs to proper thickness, the cement being used is forced into the voids of the wall panels to effectuate a concrete fusion between wall and floor components.