Chiaramonti was born at Cesena, the son of Count Scipione Chiaramonti; his mother, Giovanna, was the daughter of the Marquess Ghini and through her the future Pope Pius VII was related to the Braschi family, the family of Pope Pius VI.
On Luigi's marriage to the richest lady of the Falconieri family, he was granted permission by Pius to build Palazzo Braschi off Piazza Navona, and from 1787 and 1795 he built another neoclassical Palazzo Braschi at Terracina, as a private residence for his uncle.
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Luigi Braschi Onesti (1745 – 1816), duca di Nemi, was a nephew of Pope Pius VI, who granted him his dukedom.