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.
Andrea Bregno
The attribution to Andrea Bregno and Baccio Pontelli of the Church of Santa Maria del Popolo, commissioned by Sixtus IV, is traditional, as is the tradition that the two were responsible for the Palazzo della Cancelleria. Donato Bramante followed in both locations. In Santa Maria del Popolo, Bramante extended the apse, but the facade is of the earlier campaign, picked out in architectural guides as one of the finest pieces of Early Renaissance architecture in Rome. Most of the hexagonal side chapels with sexpartite ribbed vaults and balustrades preserved their original layout.