Abstract: A method for constructing prefabricated modular building panels includes providing a plurality of full wall height panels in several widths and full wall height corner elements which can be assembled to provide walls for a building of almost any size. The method also includes providing less than full height header and sill panels in several widths which can be used to install windows and doors. A window is installed between a header and sill panel having a width which is greater than the width of the window by placing studs between the header and sill panels at each side of the window and at each side of the header and sill panels. A door is installed under a header panel having a width which is greater than the width of the door by building a frame on each side of the door below the header panel. The panels are made with a frame of interconnected steel studs and steel top and bottom tracks.