Abstract: A method of construction by the use of prefabricated building units is described in which an open three-dimensional steel cage of relatively light structure is constructed off-site according to the desired specific configuration of the building unit; pre-cast concrete wall panels are attached to the sides of the steel cage by welding metal rings embedded within openings in the pre-cast concrete panels to laterally-projecting metal rings welded to the steel cage, to form the walls of the building unit and to add strength and rigidity to the building unit; the building unit is completed with a floor and roof further adding strength and rigidity to it; and the completed building unit is then transferred to a suitably prepared foundation at the construction site.