Abstract The built environment is the most resource intensive sector of the economy, accounting for a significant share of the extracted materials and the total waste generated. Within the built environment the most recurrent replacements of building materials and components take place during fit-outs, which are the process of installing interior fittings, fixtures and finishes. These materials and components are frequently replaced in non-domestic buildings. Non-domestic building fit-outs are therefore responsible for a significant consumption of materials and a large source of waste. However, they tend to be excluded and unmeasured in the research on the built environment. The present work aims to study this research gap and analyse the potential for fit-outs to become more sustainable. The approach of this project ties in closely to the concept of circular economy, where materials are kept at their most useful state for as long as possible. This paper analyses fit-out practices within London, identifying the supply-chain stakeholders, the key materials used and the waste streams generated, while tracing the decision and material flows across the supply chain. A material flow analysis (MFA) is conducted for a fit-out case study, showing the paths and destinations of the waste generated. The mixed methodology includes on-site observations, cross-examination of the corresponding waste reports, MFA, and qualitative analysis of interviews with the involved stakeholders. The aim of this research is to provide a grounded perspective that allows the identification of process and design flaws as well as potential improvements that support the transition towards more “circular” fit-outs. It is concluded that there are potential areas of improvement as fit-out practices show a predominantly linear tendency both for decision making and material flows, in which there is a discontinuity of communication and material-flow information across the supply chain.