The present invention provides a system, or kit, of injection molded panels having integrated connectors which combine to form an enclosure, commonly in the form of a utility shed. The panels are formed of injection molded plastic to interlock with one another without the need for separate I-beam connectors. The ends of the wall panels have cavities to accept both roof and floor outwardly projecting locking bosses for interlocking cooperative engagement which serve to rigidly connect the components together. The symmetry of the wall, roof, floor and door components also minimizes component shapes and simplifies enclosure construction.