A new approach of retrofit design methodology in cogeneration heat and power systems based on constructal theory is presented in this paper. A cogeneration system may consist of different turbines, steam levels and steam generators. The steam demand of each level is determined and should be supplied. The purpose of this paper is to retrofit the existing total site heat and power cogeneration system utilizing the concepts of constructal theory. Developing constructal theory to total site cogeneration systems may lead to divide the total site into different constructs. In this paper the total site cogeneration system will be divided into three constructs: turbines, turbine array between each two levels and steam generators array. Using constructal theory simplifies the total site complex system to a simpler system that can be solved easily by a simple search and sort method. The best configuration of the total site would have the minimum operating cost. Using constructal theory would simplify the optimization procedure of cogeneration systems in addition to reach better conceptual design especially in more sophisticated systems. The methodology is applied to a sophisticated total site heat and power cogeneration system as case study from literatures. The constructal retrofit results 14.1% and 14.3% reduction in operating cost and fuel consumption respectively.