There is an increasing demand for the limited resources of the country, and the awareness about pollution is becoming more and more growing. New technologies and knowledge represent great potential for improving the energy efficiency of buildings. Sustainable construction represents some new standards that humanity should follow to ensure a healthier and more comfortable living space. The potential of nature is at our disposal, and it is up to us to use it. It is a new view of the world that results in quality and wider application. Sustainable construction can also significantly reduce waste through responsible waste management and material selection. Using renewable energy sources such as sun, wind, and biomass to produce energy in buildings helps to avoid air and water pollution. In order to harmonize the wishes of the users, an intervention was carried out on the building of the University of the North. We extend the lifespan of old buildings by reconstruction and adaptation. Higher levels of building thermal protection reduce heating costs and improve thermal comfort throughout the building's lifetime, while certainly increasing the investment costs of the building or reconstruction. The building of the Varaždin County Pharmacy underwent a complete energy renovation - changing the external carpentry, renovation of the facade and heating, cooling and ventilation systems. Saving energy is the ultimate goal.