Method for retrofitting a building with a lift system, which building has a multitude of floors (3, 3.1, 3.2...) each with at least one floor door (4, 4.1, 4.2...), which are accessible via a common stairwell, wherein the stairwell is adjacent to a front side of the building, forming a stairwellfacade, and for each floor (3.......) has a main landing (6, 6.1, 6.2...) for the floor doors (4...) and between each of the floors (3...) an intermediate landing (12, 12.1, 12.2...) arranged on the staircase facade for a double-flight staircase, comprising the method steps: - making a foundation for a prefabricated lift system in front of the stairwellfacade, - opening the front side of the building by removing the stairwell facade, - attaching a frame to the open stairwell, wherein the frame is or will be fitted with a lift landing (18, 18.1, 18.2...) of substantially the same width as the stairwell and at the level of the respective main landing (6...) for each floor (3...) to be made accessible in a barrier-free manner, - removing the intermediate landings (12...) and at least partially dismantling the double-flight staircase, - installing bridges (19, 19.1, 19.2...) in the stairwell which extend horizontally from the main landings (6...) to the lift landings (18...) and only partially cover the surface area of the stairwell in projection, in order to leave space for a single-flight staircase, - connecting the main landings (6...) to the lift landings (18...) of floors (3...) above via a single-flight staircase, - erecting the prefabricated lift system on the foundation, - aligning the lift system on the foundation and creating connections between the lift system and the lift landings (18...) and/or the frame, characterised in that in the method step of removing the intermediate landings (12...) and at least partially dismantling the double-flight staircase, first stair sections (13, 13.1, 13.2...) of a first half of the double-flight staircase, which in each case connect a main landing ( 6...) to the intermediate landing (12...) to be dismantled, remain in the stairwell and second stair sections (14, 14.1, 14.2...) of the other half of the double-flight staircase are disassembled and turned through 180 angular degrees about the vertical axis, are attached to the respective first stair sections (13...) in order to form, together therewith, the stair sections of a single-flight staircase connecting the main landings (6...) to the lift landings (18...) of floors (3...) above or below. comprising the method steps: - making a foundation for a prefabricated lift system in front of the stairwellfacade, - opening the front side of the building by removing the stairwell facade, - attaching a frame to the open stairwell, wherein the frame is or will be fitted with a lift landing (18, 18.1, 18.2...) of substantially the same width as the stairwell and at the level of the respective main landing (6...) for each floor (3...) to be made accessible in a barrier-free manner, - removing the intermediate landings (12...) and at least partially dismantling the double-flight staircase, - installing bridges (19, 19.1, 19.2...) in the stairwell which extend horizontally from the main landings (6...) to the lift landings (18...) and only partially cover the surface area of the stairwell in projection, in order to leave space for a single-flight staircase, - connecting the main landings (6...) to the lift landings (18...) of floors (3...) above via a single-flight staircase, - erecting the prefabricated lift system on the foundation, - aligning the lift system on the foundation and creating connections between the lift system and the lift landings (18...) and/or the frame, - making a foundation for a prefabricated lift system in front of the stairwellfacade, - opening the front side of the building by removing the stairwell facade, - attaching a frame to the open stairwell, wherein the frame is or will be fitted with a lift landing (18, 18.1, 18.2...) of substantially the same width as the stairwell and at the level of the respective main landing (6...) for each floor (3...) to be made accessible in a barrier-free manner, - removing the intermediate landings (12...) and at least partially dismantling the double-flight staircase, - installing bridges (19, 19.1, 19.2...) in the stairwell which extend horizontally from the main landings (6...) to the lift landings (18...) and only partially cover the surface area of the stairwell in projection, in order to leave space for a single-flight staircase, - connecting the main landings (6...) to the lift landings (18...) of floors (3...) above via a single-flight staircase, - erecting the prefabricated lift system on the foundation, - aligning the lift system on the foundation and creating connections between the lift system and the lift landings (18...) and/or the frame, characterised in that in the method step of removing the intermediate landings (12...) and at least partially dismantling the double-flight staircase, first stair sections (13, 13.1, 13.2...) of a first half of the double-flight staircase, which in each case connect a main landing ( 6...) to the intermediate landing (12...) to be dismantled, remain in the stairwell and second stair sections (14, 14.1, 14.2...) of the other half of the double-flight staircase are disassembled and turned through 180 angular degrees about the vertical axis, are attached to the respective first stair sections (13...) in order to form, together therewith, the stair sections of a single-flight staircase connecting the main landings (6...) to the lift landings (18...) of floors (3...) above or below.