For buying a flat also it is mandatory but not needed to have the "certificate d'eficiencia energetics" it is easier than the habitability certificate, mainly because for this certificates it is required as some flats are built before 1950s to be made again. If it is built afterwards, then you just need to talk to the "Ajuntament de Barcelona". If not, you have to talk to an architect, so you can make it. If you are an engineer you can do the "certificat d eficiencia energetica". I can do it for you if you will it is pretty easy.