Abstract: In a method and system of designing a stair lift rail assembly to be mounted on a three-dimensional structure, a light beam including an optical pattern on at least part of the structure is projected from a reference location relative to the structure. Light from the structure is detected. Image data of the structure are generated based on the detected light. The image data are processed to generate a set of map data of the structure, the set of map data representing a three-dimensional map of the structure. Spatial path of the stair lift rail and locations of support interfaces for the stair lift rail assembly in the three-dimensional map are determined. Design of the stair lift rail assembly is generated based on the spatial path of the stair lift rail and the locations of the support interfaces for the stair lift rail assembly.