In accordance with Directive 2010/31/EU (EPBD recast), all EU Member States are obliged to perform analysis on cost optimal levels of minimum energy performance requirements. Besides EU Member States, this EU directive influence candidate countries correspondingly. In order to establish national calculation methodologies for cost optimal energy performance levels, EU methodology framework proposes several main stages based on national variables. This paper provides an original process to be considered for development of national calculation methodologies for cost optimality calculations. In particular, the method investigates validity of global cost calculation periods given in EU method under national market conditions. The method is applied to an office building following the EU framework method and calculations are performed for two climatic zones of Turkey which are extremely different from each other. This case study is a preliminary sample cost optimality study for Turkey in terms of EPBD recast as well. Results of the case study are explained in detail to define key points of cost optimality calculation methodology and possible problems during the application of the methodology with national factors. Analyses verify interrelation between all stages of the framework methodology. According to results, it is crucial to improve the methodology in terms of reference building definitions and global cost calculation periods throughout the adaptation procedure of national cost optimality calculation method. Furthermore, collaboration with national market is a key issue for achieving a convenient calculation methodology. The analyses also reveal that, reference building definitions, climate, cost calculation periods, amount of capital and unsteady financial rates have considerable influence on cost optimal analysis at national level. (C) 2014 Elsevier Ltd. All rights reserved.