A modular construction assembly comprising a plurality of three-dimensional construction elements connected together to form a wall, ceiling or floor; wherein adjacent three-dimensional construction elements are fastened together by welding and/or bonding and/or mechanical fasteners; wherein each three-dimensional construction element is assembled from a multi-panelled metallic sheet, and wherein the metallic sheet is rectangular in shape and formed from a steel plate having a thickness of between 6mm and 25mm; wherein each three-dimensional construction element comprises only a base panel and two sidewall panels (16,18) which together define a U-shaped channel; and wherein the base panel of one U-shaped channel is fastened along distal edges of both sidewall panels to an adjacent U-channel-shaped constructional element so as to form a lid closing the open top of the adjacent U-shaped channel; wherein the planes of adjacent panels are mutually perpendicular; and wherein each construction element, in a pre-assembly condition comprises said multi-panelled metallic sheet sub-divided by one or more fold lines (12a, 12b) into panels (14, 16, 18) to define said multi-panelled sheet wherein each panel lies in a common plane; at least one of the panels being deformable along said one or more fold lines out of said common plane to form an assembled three-dimensional construction element for adjoining to another three-dimensional construction element; and wherein at least one panel of the multi-panelled metallic sheet is provided with an opening dimensioned to allow the passage of a reinforcement or stabilising material through the assembled three-dimensional construction element. wherein adjacent three-dimensional construction elements are fastened together by welding and/or bonding and/or mechanical fasteners; wherein each three-dimensional construction element is assembled from a multi-panelled metallic sheet, and wherein the metallic sheet is rectangular in shape and formed from a steel plate having a thickness of between 6mm and 25mm; wherein each three-dimensional construction element comprises only a base panel and two sidewall panels (16,18) which together define a U-shaped channel; and wherein the base panel of one U-shaped channel is fastened along distal edges of both sidewall panels to an adjacent U-channel-shaped constructional element so as to form a lid closing the open top of the adjacent U-shaped channel; wherein the planes of adjacent panels are mutually perpendicular; and wherein each construction element, in a pre-assembly condition comprises said multi-panelled metallic sheet sub-divided by one or more fold lines (12a, 12b) into panels (14, 16, 18) to define said multi-panelled sheet wherein each panel lies in a common plane; at least one of the panels being deformable along said one or more fold lines out of said common plane to form an assembled three-dimensional construction element for adjoining to another three-dimensional construction element; and wherein at least one panel of the multi-panelled metallic sheet is provided with an opening dimensioned to allow the passage of a reinforcement or stabilising material through the assembled three-dimensional construction element.