Much of what is known about Roman building practices derives from the study of structures built between the second century BCE and second century CE. Shifting our focus to the late Roman Empire reveals very different practices at work. In the third and fourth centuries CE, there are far fewer of the freshly quarried and custom requisitioned architectural marbles that had dominated Imperial building a century earlier. In their place, reuse emerges as a normative architectural practice, even for the most prestigious imperial commissions. Carved architectural elements and relief sculpture appear in secondary contexts as a matter of course, and architectural designs adapt in response to reused materials. Yet reuse in late Imperial Rome also extends to the building sites themselves: by the Tetrarchal period, almost every major imperial commission is built atop and / or adjoining another structure. Using late third and early fourth century imperial commissions in Rome as a case study, I outline the evidence for this shift in architectural practice, and argue for a new appreciation of the complexities of designing and building in a city that had seen over 1000 years of continuous habitation and four centuries of dense urban construction in concrete.