The subject of this master thesis is a holistic energy rehabilitation of the building of the Veno Pilon Ajdovščina Secondary School. In accordance with the key principles of building, our renovation plan proposes a sufficient thermal and sound insulation, waterproofing of the building and daylight illumination inspection of some of the chosen areas. The first step was a thorough examination of the building and the evaluation of the current state of the building and its inbuilt elements and materials. Furthermore, we checked their relevance according to the current legislation and identified the critical elements that had to be rehabilitated or upgraded. The KI Energija 2014 and the Hrup'13 programs were used for the analysis of the individual elements. An analysis of the reverberation time of indoor environment was calculated for two spaces with the ARTIE software. Afterward, we analysed the climate conditions of the location and also performed a parametric study of the heat transfer coefficient of the building envelope. Appropriate structural assemblies that now meet the requirements were designed as well. Designing the transparent surfaces of the roof, we used the Velux Daylight Visualizer software to analyse the effect these surfaces have on the internal illumination of the selected spaces. Lastly, we performed an analysis of the energy efficiency and the energy consumption with the use of the KI Energija 2014 software. At the end, we presented the results, commented on them and added some additional propositions to further improve the energy efficiency of the building.