Improvement of energy efficiency applications through public buildings can play a leadership role demonstrating best practice in environmental sustainability for the wider population. Therefore, this study was conducted on an educational building stock in Istanbul for a year. A selected energy classification method was applied in order to designate the energy classes and related future target level assumption, which was assigned by EPBD Recast (2010). A representative base case from the stock was determined to analyse the cost-optimal levels of the energy retrofits. GenOpt optimization tool was used to apply the energy retrofit scenarios in order to achieve a set of optimum scenarios in terms of energy and cost (energy frontiers) and determine the cost-optimal energy efficiency level through a calibrated energy model. Finally, results were evaluated from energy efficiency and cost-optimal point of views.