Educational buildings in Europe account for around 20% of the entire non-residential floor space where good indoor comfort and air quality are essential for correct educational development. Directive 2010/31/UE upholds that the public administration should promote the transformation of its buildings towards Net Zero Energy Buildings. In Spain, it is particularly important to act on the existing building stock since the construction of new buildings has been considerably reduced as a result of the bursting of the property bubble following the frenzied building activity over the last few decades. This article proposes a method to define and assess strategies to achieve NZEB in university buildings based on student comfort analysis under real conditions. A questionnaire and monitoring campaign was conducted in a typical spring week in the Architecture Faculty in San Sebastian (Spain). User preferences have been considered as an energy saving opportunity. The analysis revealed that students prefer lower indoor temperatures (20-22.5 degrees C) than stated by theoretical comfort models. This was the starting point to analyze in detail strategies to reduce the energy consumption of the building and above all, to prioritize these strategies by impact and significance. The principal measures that make it possible to achieve comfort conditions work together with energy saving strategies, which can be achieved through effective interventions in the building. Retrofitting strategies for the winter period are: eliminating thermal bridges, using air-to-air heat recovery systems and improving the windows in the north facade of the building. The results show a potential energy saving of up to 62% and a reduction of two months in the heating period for the Faculty of Architecture. Furthermore, overheating problems reported by users in summer and shoulder seasons could be solved by using 4 ACH day and night time ventilative cooling, avoiding the installation of air-conditioning systems and all the associated environmental impacts. Finally, acting on the existing building stock implies necessarily understanding user needs in order to define most adequate energy saving strategies. (C) 2017 Elsevier B.V. All rights reserved.