This article explores the complexities of informal urbanisation at the metropolitan periphery of Mexico City through a case study of Ampliación San Marcos, a former agricultural area on the city's south-eastern periphery. While the physical annexation of small towns and their environs is a common feature of Mexico City's growth, the settlement of Ampliación San Marcos is more accurately described as a two-pronged process involving the extension of a nearby pre-Hispanic town and the expansion of Mexico City itself. The case study shows that the rural periphery of Mexico City is no tabula rasa upon which urban growth simply 'takes place', rather, settlement processes are influenced by longstanding in situ social relations and practices related to property. The paper highlights the importance of considering the relationships among social relations, property and informal settlement for understanding the complexity of metropolitan growth and change in large cities such as Mexico City.