Well into the seventh century, masons in Rome built bonded-masonry walls using materials and techniques directly descended from antiquity. But walls erected starting in the eighth century are very different and distinctively ‘medieval’. The late seventh / early eighth century therefore represents a moment of rapid transition or even rupture in the Roman building industry, when older ways of doing things ceased forever. Drawing on recently excavated structures on the Palatine and at San Paolo fuori le Mura that offer new insights into this crucial transitional period, I suggest that the break with centuries-old building traditions reflects a fundamental shift in mechanisms of patronage, and of control over the city's built environment. After a hiatus in the second half of the seventh century, when the Roman construction industry languished between a Byzantine administration in decline and a Church bureaucracy not yet empowered to supplant it, early eighth-century popes faced the challenge of creating anew the means and methods to build on a substantial scale. The newly excavated structures of the early eighth century offer an unexpected perspective on the growth of, and the growing pains experienced by, Rome's nascent papal government.