The nZEB (nearly zero energy building) requirements taking effect in 2021 have induced the interest about additional cost and technical solutions amongst real-estate developers, construction companies, architects and engineers. The objectives of this study are twofold, first to determine the cost-optimal energy efficiency level for two lately built apartment buildings in a cold climate of Estonia, and secondly, which are the proper measures to achieve low energy and nZEB requirement levels. The influence of high-efficiency external walls, roofs, windows, ventilation units and solar collectors on energy use and construction costs were studied by using multi-stage methodology for reducing the number of combinations. The results show that since 2010 the cost optimal primary energy level has shifted from 145 kWh/m(2) to 110 kWh/m(2), but achieving nZEB level of 100 kWh/m(2) still requires relatively high additional investments. With initial measures, nZEB requirements were not fulfilled in the studied cases, but the solutions close to nZEB required extra investment of 65 /m(2) i.e. 4-7% compared to the actual buildings. Low energy building level needed additional investments up to 2%. In both case studies remarkably better energy efficiency level could have been achieved with lower construction costs. (C) 2015 Elsevier Ltd. All rights reserved.