Directive 2010/31/EU (EPBD recast) introduces the concept of nearly zero energy building (NZEB) stating that all new buildings must be NZEB by the end of 2020. According to the Directive, a NZEB is a building that has a very high energy performance and its energy need is covered to a very significant extent by energy from renewable sources (RES). It is up to each Member State specify the meaning of “very high performance” and “very significant extent”. The same Directive mentions cost optimality as a driver in fixing building energy requirements. Particularly, the Commission established a comparative methodology framework for calculating cost-optimal levels of minimum energy performance requirements for building and buildings elements. So, NZEB energy performance should be equal or better than the cost-optimal level calculated with the comparative methodology applied in 2020. In this paper we define two 2020 scenarios and then we apply the comparative methodology in order to derive the cost-optimal levels, i.e the energy performance, in terms of primary energy, leading to minimum life cycle cost. These levels are benchmarks for NZEB definition. In the first scenario we use current input values, with a slight improvement in the energy efficiency of building systems and a reduction of the primary energy and CO2 factors for electricity. In the second scenario, in addition to the previous hypotheses, we assume cost reductions of RES technology and of new generation systems due to technological progress. The study is focused on new residential buildings (two typologies) and takes into account the effect of two different climate conditions (northern and southern Italy). The energy efficiency measures considered cover different levels of thermal insulation, double and triple glazing, shading devices, condensing boilers, air and ground source heat pumps, combined heat and power, photovoltaic and thermal solar collectors and mechanical ventilation with heat recovery.