A modular constructed building includes a plurality of ground beams and a plurality of wall panels, each ground beam has a pair of surfaces and a longitudinally extending male projection interspaced between the pair of surfaces. The extending male projection is formed as a flat plate and at least able to be partially inserted into a wall panel with each of the wall panels being secured to a respective ground beam, so that each wall panel, at an end opposite the ground beam, has a lateral groove for accepting a protrusion for a T-shaped member or an X-shaped member, and an upper end of each of the wall panels is retained together through each of the wall panels and either a T-shaped member or an X-shaped member.