Sustainable building has become a subject of high policy emulation to address sustainability challenges. Publications and interventions from international organisations concord in outlining the high share of greenhouse gas emissions produced by the building sector, while at the same time pointing towards the important leverage for sustainability interventions on the built environment. The realisation of sustainable building crosses serval functional domains: regulatory, economic, cultural, social, natural etc. It requires also to consider buildings in a comprehensive manner from their inception, construction, up to their use and eventual retrofit or demolition but also in relationship with their wider (urban) settings. And last but not least, it requires close interactions with a wide range of stakeholders with different interests and disciplinary perspectives. As a result, sustainable building transformations are a widely diversified agenda. Hence the primary interest, as well as objective of this work, has been to try to better grasp the causes and mechanisms that explain such large differentiation of sustainable building. Drawing on critical sustainable urban literature, I posit sustainable building as diversified because it is a situated and socially mediated object. Conceptually, the work engages with two approaches. First the literature on sustainable socio-technical transitions, in particular the Multi-Level Perspective. Its grounding in co-evolutionary and institutional thinking helps to comprehend the socio-material complexity of sustainable building across a broad range of dimensions and connections. In complementing, and following the adopted socially constructed stance, the work further looks at the discursive constructs used to argue in favour of the transformation towards sustainable building, as the performativity of discourse helps to explain why that change occurs. The operationalisation of the research inquires sustainable building transformations in two European urban areas, Luxembourg (LU) ...