Abstract: Disclosed is a modular building system and method of building an expandable building. There is a visible foundation comprising structural beam elements of reinforced concrete having a plurality of foundation rods extending upwardly therefrom and a plurality of modular blocks for forming walls of the building extending upwardly from the foundation beam elements. The blocks have vertical apertures therethrough and are adapted to be assembled into the walls such that a plurality of vertical ducts are formed in the walls by the apertures of adjacently assembled blocks. Wall rods having upper and lower ends with the lower ends of the wall rods being connectable to the foundation beam rods at selected locations thereof and adapted to extend upwardly through selected ones of the assembled wall ducts. At least some of the wall rods are of a height where the upper ends are exposed above an assembled wall.