We report a case study on the conversion into a Nearly Zero Energy Building (NZEB) of an existing building, “The Visitors Centre for Nature Interpretation “located in the Pyrenees mountains of Catalonia, Spain. The original building was constructed in the last decade using strategies of passive and low energy buildings. At that time, there were no regulations about energetic certification in our country, and we have no reliable information about the energetic performance of many modern buildings theoretically considered as “low energy”. The research was carried out by collecting data on real consumption and CO2 emissions values and analysing both the energy systems and the passive conception of the building. Afterwards, our conclusions furnished information on which to choose the strategies aiming at the energetic refurbishment and the conversion into nearly NZEB of this public centre. The main finding is that, by applying a combination of refurbishment options and improving the energy systems, a saving of approximately 50% is achievable in energy use and carbon emissions. Due the optimal levels of solar radiation in this place, the rest of the energy needed to afford the NZEB goal can be supplied by a photovoltaic installation placed on the roof. This approach can be applied to inspire and inform future reconversions of the existing modern buildings in Catalonia into nearly NZEBs.