European regulatory efforts, towards increasing energy efficiency of buildings, are focusing on a comparative methodology framework for calculating cost-optimal levels of minimum energy performance requirements for buildings and building elements, which has been published as a European regulation 244/2012. The framework methodology requires that many parameters have to be decided on national level. The paper describes influences of parameters and the change on final requirements. In different examples of the cost optimal method indications coming from guidelines accompanying Commission Delegated Regulation (EU) No 244/2012 (2012/C 115/01) and standard CEN EN 15459 “Standard economic evaluation procedure for energy systems in buildings” will be taken into account. In the paper the results obtained using the framework for two different assumptions in order to define the energy performance requirements for building are presented. The aim of the paper is opening the discussion on cost optimal methodology and turning on the attention for the needs for further harmonisation of the approach. The presented considerations reflect the works of the IEE project on Market Transformation Towards Nearly Zero Energy Buildings Through Widespread Use of Integrated Energy Design.