The article presents the general principles of flexible approach, illustrated with case study regarding flexibility in modernization. Flexibility is understood in the present case as a skilful adaptation to changes in the dynamically changing environment. The essence of flexibility approach is being staged to make decisions based on environmental and process monitoring in progress. Presented case study of upgrading the electrical system in the multi-family housing in the common area is based on step by step procedure responding to damage state of this system. Simple analysis is based on cost comparison but there are other criteria (environmental, social etc.) and conflict solving situations taken into account in presented paper.