Hello, as you may be aware that regions in Spain have different rules. I am sure Asturias is no exception.
As a rule if the foot prints of the building are not altered. it should be straight forward. As you may also be aware that nothing is straight forward in Spain. The reasons could be the last works done may have been illegal, the planning department or other department making money out of you, others invloved maning money out of a confusion or creating confusion due to a disjointed system.
If I was you. I would speak to one or two reputable gestors.