hmm, personaly I should not spend money to ask this to a lawyer. I think nobody can tell you what your position will be in this case. Your question is about how the authorities will act when you resell -in the future-. I can give you the "lawyer-like" answer right now: if the 'ruines or buildings are still exist on the registration papers there is a probability you will get a licence to refurbish them easier. But in reality nobody can tell you the law in the future and nobody can tell you if the ayuntament at that time will investigate the existance of the buildings or just follow the paper registration or demands a architectural declaration or whatever procedure will come up -in the future-.