Abstract: A building panel system includes a plurality of full size panels and half-size panels mountable to each other for building a wall. The panels each include a longitudinally extending rectangular parallelepiped elongate horizontal member. The full size and half-size panels have substantially equivalent height and depth dimensions. The full size panels each include a longitudinally spaced apart pair of vertically extending hollow pilaster columns formed on a rear face of the horizontal member. The pair is spaced apart so as to position each column of the pair towards opposite ends of the horizontal member. The half-size panels each include a single vertically extending hollow pilaster column formed on a rear face of the horizontal member. The pilaster columns extend vertically by the height dimension.