Santa Maria del Popolo
The architect or architects of this innovative project remain unknown due to a lack of contemporary sources. Giorgio Vasari in his "Lives" attributed all the important papal projects in Rome during Sixtus IV to a Florentine, Baccio Pontelli including the basilica and monastery of Santa Maria del Popolo. Modern researchers deemed this claim highly dubious and proposed other names among them Andrea Bregno, a Lombard sculptor and architect whose workshop certainly received important commissions in the basilica. The fundamental differences between the façade and the interior suggest that maybe more than one architect was working on the building.
Lateran Palace
Two destructive fires, in 1307 and 1361, did irreparable harm, and although vast sums were sent from Avignon for the rebuilding, the palace never again attained its former splendour. The palace had Gothic architectural elements at this point. When the popes returned to Rome they resided first at Basilica di Santa Maria in Trastevere, then at Basilica di Santa Maria Maggiore, and lastly fixed their residence at the Vatican. St. Peter's Basilica, also built by Constantine, had until then served primarily as a pilgrimage church. Sixtus V, more concerned with rationalized urban planning than the preservation of antiquities, then destroyed what still remained of the ancient palace of the Lateran in 1586 preserving only the Sancta Sanctorum, and erected the present much smaller edifice in its place, designed by his favorite architect Domenico Fontana.