An upper stackable panel is provided for use with lower panels having a structural panel to panel, connector mount located at least at each upper corner of said lower panel, to which a panel to panel connector can be secured by at least one fastener. The upper panels have a structural panel mounting member located at least at each bottom corner, at a location adjacent an upper corner panel to panel connector mount when the upper panel is positioned on top of the lower panel. Each lower corner panel mounting member has a fastener configuration which is compatible with that of the adjacent upper corner panel to panel connector mount on the lower panel. At least one fastener secures each said bottom corner panel mounting member to its adjacent upper corner panel to panel connector mount, to create a structural connection between the upper and lower panels at each of their respective lower and upper corners.