One of the possible solution for enlarging useable space for existing building is to add stories. Adding new stories involves additional loadings to old structure and to subsoil. Very often calculation and analysis of existing structure and overloading of soil leads to conclusion, that only very lightweight superstructure can be applied. One of such structure can be pre-fabricated steel frame-panel SUNDAY system^TM which includes pre-engineered wall panels, roof trusses, headers and joists in fabrication shop from light-gauge steel stud made on the rollform machine. The combination of only 2 shapes (C and U) in 2 sizes each, with addition of 4 kinds of screws and using different steel gauges for these shapes is able to totally fulfill structural requirements of any structure type specified by the designer for such use. The paper present the case study of application of light steel prefabrication technology for enlargement of existing building by two additional stories.