This is not true either: 14,000 metres is the minimum plot but there have been transitional rules which at certain times allowed licences to be issued.
On the other hand, for some years a law called the amnesty, allowed to legalize all those constructions built without license that did not have open files of infraction. Therefore, a house built on a plot of land of less than 14,000 meters could have been legalized during this period.
Apart from what has been explained. Any construction on a rustic plot of less than 14,000 square meters is highly suspected of being illegal and only by checking the licenses can one know.