The Seminary building is a construction of the late eighteenth century which was opened in 1788. Although the author is unknown, it is sometimes referred to the workshop of Luigi Vanvitelli (1700-1773). It arose on some Sandri family houses and other private buildings, purchased for this very purpose, over several decades, using the assets of the suppressed monastery of St. Stephen.

It now has a three stories facade, divided into five bands, the middle one slightly overhanging; to the left, perpendicular to the main body of the building, another band advances, where the chapel and the headquarters of the former library lie. Inside, some salons come in succession, which once served as a studio and dormitories for seminarians, while both on the top floor and at the back, there are the rooms of the professors and many service quarters.