A two-piece, flush-mounting box designed for installing plumbing elements consists of the flush-mounting box itself and a flange that may be fastened to the side walls of the flush-mounting box at a certain distance from its front. That distance is preset prior to installation in order that, as the case may be, removal of its protruding portion will be unnecessary or the front surface of the flange will be flush with the front or rear surface of the wall when it has been installed.