Well, there is no protection of the existing state without a building permit ever having been granted. The mere duration of the building’s existence is not sufficient – the building must have been approved at some point for protection of the existing state. To my knowledge, there is no statute of limitations for illegal construction either. As a rule, if tolerated at all, it is tolerated as long as it is inhabited. However, you are now also in a situation where the house has not been inhabited for years and has accordingly fallen into disrepair.
A glimmer of hope in my view would only be if the building authority had, at some point since the building was constructed, (in writing) indicated in some form that it would not take action against the illegal construction and that it would accept the development permanently. However, the chances for that are most likely poor. This is probably where a lawyer comes into play, who surely knows better how certain documents can be interpreted in that regard.