A composite construction element or panel system for use in construction of multi-storey structures, either as a prefabricated panelized system or as a modular system. The composite construction element comprises two mass timber subpanels joined at a distance to form a hollow core, through which various building services (e.g. HVAC or electrical systems) and/or insulation may be integrated. When used as a modular system, after assembly, the two subpanels become one structural entity of increased structural capacity while providing a hollow core to provide/deliver desired building services.