The present invention pertains to a modular furniture system in which a flat-foldable or easily assembled and disassembled rigid base structure composed of rectangular-cross-sectioned pieces each with specific dimensions serves variously, once assembled, as the supporting framework for a flat-foldable chair seat-and-backrest module, which configured together with the base structure serves as either an upright dining chair with armrests or as a lower and more reclined lounge chair with armrests, or alternatively as configured for supporting the connectable or foldable components of a flat planar surface module to be used for dining or to serve as a desktop, or alternatively as configured for supporting a storage cabinet module with drawers or doors.