Method for retrofitting a building with a lift system, wherein the building contains a plurality of stories which are in particular subdivided and have story doors which are accessible via a common staircase, wherein the staircase is adjacent to a front side of the building and, for each story, has a main landing for the story doors and an intermediate landing for a double flight of stairs, comprising the method steps of: - prefabricating the lift system with a shaft, with a cabin which is movable in a longitudinal direction of the shaft, with the guides and drive mechanisms of said cabin, and in each case with a lift landing for bridging a distance between the cabin and the building for each story, wherein the shaft extends over at least two stories and comprises a self-supporting sheet metal box made of at least one steel plate which is provided for receiving the cabin together with its guides and drive mechanisms, which steel plate extends in a straight line in the longitudinal direction of the shaft while being profiled in a cross-sectional plane orthogonal to the longitudinal direction of the shaft, and wherein the sheet metal box of the shaft has a cross section which forms, in the cross-sectional plane within the sheet metal box, a cross-sectional area substantially containing the area required by the cabin and its guides and drive mechanisms and additionally the area of the lift landing, - fabricating a foundation for the lift system in front of the front side of the building, - opening the front side of the building in the width of the staircase, - removing the intermediate landings and the double flight of stairs, - installing walkways at the height of the main landings, which walkways extend horizontally from the main landings to the front side of the building and thereby only partially cover the cross section of the staircase to provide clear space for a flight of stairs, - installing a single flight of stairs which connects the individual walkways to one another and forms a modified staircase therewith, - placing the lift system on the foundation, - aligning the lift system on the foundation and joining the lift system to the front side of the building, - establishing a connection between the lift landings and the walkways on each story, - establishing access to the lift landing of the ground floor from outside the building. - prefabricating the lift system with a shaft, with a cabin which is movable in a longitudinal direction of the shaft, with the guides and drive mechanisms of said cabin, and in each case with a lift landing for bridging a distance between the cabin and the building for each story, wherein the shaft extends over at least two stories and comprises a self-supporting sheet metal box made of at least one steel plate which is provided for receiving the cabin together with its guides and drive mechanisms, which steel plate extends in a straight line in the longitudinal direction of the shaft while being profiled in a cross-sectional plane orthogonal to the longitudinal direction of the shaft, and wherein the sheet metal box of the shaft has a cross section which forms, in the cross-sectional plane within the sheet metal box, a cross-sectional area substantially containing the area required by the cabin and its guides and drive mechanisms and additionally the area of the lift landing, - fabricating a foundation for the lift system in front of the front side of the building, - opening the front side of the building in the width of the staircase, - removing the intermediate landings and the double flight of stairs, - installing walkways at the height of the main landings, which walkways extend horizontally from the main landings to the front side of the building and thereby only partially cover the cross section of the staircase to provide clear space for a flight of stairs, - installing a single flight of stairs which connects the individual walkways to one another and forms a modified staircase therewith, - placing the lift system on the foundation, - aligning the lift system on the foundation and joining the lift system to the front side of the building, - establishing a connection between the lift landings and the walkways on each story, - establishing access to the lift landing of the ground floor from outside the building.