A modularly constructed storage structure with mezzanine access. The storage structure is provided in the form of a row of storage cabinets stacked on top of each other thereby providing base cabinets and upper storage cabinets. The storage cabinets have internal supporting structural framework which include vertical front and back beams and horizontal side beams with the frameworks of the stacked cabinets interconnected, and the vertical back beams of the base cabinets are also secured to an underlying supporting floor. The mezzanine structure, which is attached to the cabinet structure, includes a plurality of spaced horizontally aligned cantilever beams of predetermined length which penetrate the storage structure and are respectively connected directly or indirectly to front and back vertical beams of the cabinet structure. Overhang ends of these cantilever beams extend beyond the front face of the cabinet structure for cantilevered support thereon of a mezzanine deck. Decking is supported on these beams overhangs for mezzanine access to the upper storage cabinets. Since the mezzanine structure is entirely supported in cantilever fashion, no underlying vertical supports are required. A stairway may be attached as desired.