Since buildings account for 40% of total energy consumption and 36% of CO2 emissions in the EU, the directive 2010/31/EU “Energy Performance of Buildings Directive (EPDB)” among other key laws concerning the reduction of energy consumption of buildings has been enforced. According to this legislation all new buildings must be nearly zero energy buildings “nZEB" by 31 December 2020 (public buildings by 31 December 2018). Nonetheless the assessment of the “high energy performance” of a building is ambiguous and a cross country comparison seems to be intricate as far as the different national building codes employ different energy indicators. This thesis delves into the question of how do the “nZEB” definition and the transposition of the Directive 2010/31/EU into national law change in four selected EU Countries: Austria, Germany, Spain and England. The energy performance of some exemplary buildings is assessed by means of a simplified MATLAB model based on the norm DIN V-18599. The results drawn from this work show how diverse are building codes scopes and national “nZEB” definitions. Only 9 of the 36 studied cases of residential buildings obtain consistently the “nZEB” status in all four selected countries. Different climate conditions, energy requirements, primary energy factors, ambition levels and calculation methodologies lead to the problem of an uneven cross-country comparison. Moreover, primary energy consumption [/2] set as the main quantitative energy indicator by the directive 2010/31/EU might not be the most suitable one for an EU level comparison. An EU level nZEB definition grounded on the combination of two measures, namely (1) the set of an absolute value for the maximum energy need for heating and cooling in [/2] with a correction factor depending on the climate zone at EU level and (2) the set of a relative maximum value [%] for the primary energy consumption in regard to a reference building, could support and ease the task of the projects and initiatives intended to provide data and input on how to reach the nZEB standard and hence encourage the compliance of the proposed energy consumption reduction and CO2 emission reduction targets.