A prefabricated extension module and complementary anchorages for cantilever attachment to a building façade, comprising: a rigid floor panel integrating a structural floor frame, and including a flat top surface, an inner edge provided to be arranged facing the building façade when the module is attached thereto, two side edges perpendicular to the inner edge and an outer edge comprised between the side edges; two rigid side panels, each integrating a structural side frame, and being provided with a bottom edge, a top edge, and an inner edge provided to be arranged facing the building façade when the module is attached thereto, each of said side panels having the bottom edge thereof hinged to the floor panel about an axis parallel to the side edges of the floor panel by means of a hinge coincident with or adjacent to one of the side edges, allowing the side panels to be positioned in a horizontal position parallel to the flat top surface of the floor panel and in a vertical position perpendicular to the flat top surface of the floor panel; whereby the complementary anchorages are at least one first anchorage and at least one second anchorage, and wherein the at least one first anchorage is screwable to a building façade; characterized in that the at least one first anchorage is provided with first coupling configurations, formed by multiple parallel elongated profiles projecting from the first anchorage in an direction almost parallel with respect to the side edges of the floor panel or forming an upward inclination, the first coupling configurations being complementary to and insertable into first receiving configurations contained in the floor panel and accessible through the inner edge thereof, the first receiving configurations are elongated housings or aligned successions of housings structurally integrated with the structural floor frame, each elongated housing or aligned succession of housings are configured for the tight insertion of an elongated profile of the first coupling configurations in a direction perpendicular to the façade; the at least one second anchorage is screwable to the building façade above the first anchorage, and is provided with second coupling configurations connectable directly and/or through an interposed element to the two side panels by means of second receiving configurations complementary to said second coupling configurations and into which the second coupling configurations are insertable when the side panels are in the vertical position; and wherein each hinge is structurally fixed to the structural floor frame and to the structural side frame and sized for the transmission of structural stresses between them. a rigid floor panel integrating a structural floor frame, and including a flat top surface, an inner edge provided to be arranged facing the building façade when the module is attached thereto, two side edges perpendicular to the inner edge and an outer edge comprised between the side edges; two rigid side panels, each integrating a structural side frame, and being provided with a bottom edge, a top edge, and an inner edge provided to be arranged facing the building façade when the module is attached thereto, each of said side panels having the bottom edge thereof hinged to the floor panel about an axis parallel to the side edges of the floor panel by means of a hinge coincident with or adjacent to one of the side edges, allowing the side panels to be positioned in a horizontal position parallel to the flat top surface of the floor panel and in a vertical position perpendicular to the flat top surface of the floor panel; whereby the complementary anchorages are at least one first anchorage and at least one second anchorage, and wherein the at least one first anchorage is screwable to a building façade; characterized in that the at least one first anchorage is provided with first coupling configurations, formed by multiple parallel elongated profiles projecting from the first anchorage in an direction almost parallel with respect to the side edges of the floor panel or forming an upward inclination, the first coupling configurations being complementary to and insertable into first receiving configurations contained in the floor panel and accessible through the inner edge thereof, the first receiving configurations are elongated housings or aligned successions of housings structurally integrated with the structural floor frame, each elongated housing or aligned succession of housings are configured for the tight insertion of an elongated profile of the first coupling configurations in a direction perpendicular to the façade; the at least one second anchorage is screwable to the building façade above the first anchorage, and is provided with second coupling configurations connectable directly and/or through an interposed element to the two side panels by means of second receiving configurations complementary to said second coupling configurations and into which the second coupling configurations are insertable when the side panels are in the vertical position; and wherein each hinge is structurally fixed to the structural floor frame and to the structural side frame and sized for the transmission of structural stresses between them.