In Multi commercial complexes, an aspect of "extension" is important because natural demands for distinctive characteristics were "openness" and "extension". From general point of views, commercial space must provide openness. Therefore, commercial Multi-Complexes as a representative of all commercial facilities must bring openness. So, extension, the main subject of compound activity was not each element, it was rather a "common space" that acted as a medium. Common space, unlikely to space for exclusive use, its use was not pre-determined for functions, and possibilities for compound activity; expansion was actually decided by external factors. From this, this study explained how complex commercial facilities formed "extension" by identifying the progression of "compound activity", the most distinctive feature of complex commercial facilities, process of compound activity through the elements of function, space, and traffic line, and forming "extension" based on "common space utilization". From this study, we also investigated on the method of extension by identifying a relation between traits of common space extension and other areas. And common space utilization was concluded as a method with the most influential effect. This study that suggested extension methods and its reflection of flexibility through common space utilization brings meaningfulness since it provided an essential, refreshing view on complex commercial facilities.