The recast of the EPBD in 2010 (Directive 2010/31/EU) includes a provision that national energy performance requirements should be set with the view to achieving cost optimum levels by applying a harmonised calculation methodology. The Commission requests Member States to use and apply this methodology to calculate the required cost-optimal levels for their specific country and compare them with the national requirements they have set in their national building regulations. The methodology is discussed and refined in the comitology process that started in the second semester 2010. This publication has been created in support of this process. It is targeted at all internal and external stakeholders of this process and explains the background, process and generic principles of the methodology.