The object of this paper is to communicate the identification and survey of an unheard construction drawing in the wall of the Colegio de Nuestra Señora de la Antigua in Monforte de Lemos, also known as Colegio del Cardenal. In addition, the paper explores its connection with another construction drawing in the building, with the historical documents of the works and with masonry treatises. Finally, there is a reflection about the uniqueness of this kind of heritage entities and a suggestion for some actions to protect and increase its appreciation.