Abstract: Disclosed is a modular construction system that includes a base foundation having pairs of anchor straps molded therein for defining intersecting vertical wall mounting surfaces and attachment points. A plurality of panels connect between pairs of the attachment points and extend upwardly therefrom, one side of each panel being coplanar with a corresponding one of the mounting surfaces, adjacent coplanar pairs of the panels being connected by a plate that extends in line with top peripheral extremities of the panels. Adjacent sides of the panels have a tongue-and-groove configuration between the panel surfaces for holding the panel sides aligned each other in a direction normal to the panel surfaces. Angle brackets connect adjacent pairs of coplanar panels at intersecting pairs of the mounting surfaces. Joists connect between spaced pairs of the plates.