This research presents a comprehensive cost-optimal approach for existing building retrofits. The approach incorporates energy efficiency measures related to building envelope, building energy systems and renewable energy, all at once. Moreover, the cost-optimal calculation scope is extended through integration of occupant behaviour. The comprehensive approach is implemented for a reference building which represents high-rise apartment blocks in Turkey; these building types accommodate 4500 families corresponding to 23.1% of the national households. Three different climatic regions, that show significant distinction from each other, were selected for the sample application of the approach in order to refer different climates. Cost optimal calculations were performed for more than 1300 scenarios. Integration of occupant behaviour into this calculation procedure was demonstrated with sensitivity analyses; the present application of the approach was focused on the use of window openings by occupants. Results reveal that cost-effective energy saving potential is higher than 70% in high-rise apartments. Moreover, occupant behaviour related to window openings is able to raise this rate above 80%. The findings indicate the necessity of comprehensive approach and demonstrate that integration of occupant behaviour into cost-optimal calculations is fundamental in order to reach significant and stable results. (C) 2017 Elsevier B.V. All rights reserved.