Abstract: A building construction system for creating a building structure upon a foundation slab having a top surface. An inner and outer rail are secured to the foundation slab. Walls are created upon the foundation slab including interior panels and exterior panels that are secured to the inner and outer rails respectively, and corner assemblies secured to both the inner and outer rails that together define a vertical interior space therebetween. The interior and exterior panels include standard, window, and door panels that together define a vertical interior space within the walls that is partially or fully filled with a structural fill such as concrete. Ceiling joists extend upon and are supported by the interior and exterior panels, to support ceiling panels. Rafter assemblies extend upon and are supported by the ceiling joists to support roofing panels.