[2] During this period, Costaguti renovated the building to a design by Carlo Lambardi (years 1597–1619):[3] while the oldest wing of the building faces Via della Reginella, with a beautiful portal that was once the main entrance, a new entrance was opened in Piazza Mattei, in front of the famous Fountain of the Turtles; the façade here is attributed to Ascanio de Rossi, while the one in Piazza Costaguti is said to be by Antonio De Battistis.

During World War II, Marquis Achille Belloso Afan De Rivera, a lifelong fascist[4] and an officer of the Militia, during the persecutions in Rome gave shelter to many Jews in his palace: during the rastrellamento del Ghetto, «in an officer's uniform, he blocked the Germans at the doorway who wanted to enter the palace», probably to capture those who had taken refuge there from the Piazza Costaguti door to exit through the other door.

On the corners is a decoration of rusticated stone blocks from the floor to the eaves, just like the façade of Piazza Mattei, whose portal, the main entrance to the palace, is surmounted by an architrave with the inscription COSTAGUTI.

A side façade of the palace, still in Piazza Costaguti, faces the Tempietto del Carmelo, dedicated to Our Lady of Mount Carmel.

Also of note are works by Francesco Albani (Rape of Europa), the brothers Giacomo and Pierfrancesco Mola (Bacchus and Ariana) and Giovanni Lanfranco and Sisto Badalocchio (Hall of Hercules).