Many standards, tools and references are available to approach and assess the sustainability in the built environment. However, most of the available tools are strongly rooted in the technical rationality and neglect and avoid the complexity of cities. Within the overall problematic of attaining sustainability in in the built environment, this paper explores the role assessment in shaping our collective understanding of sustainability, the importance of understanding the gap that exists between the perceptions and priorities of regional users and experts, and highlights how this understanding can move us towards regional sustainability. The works of various system and complexity thinkers, social theorists, and sustainability theorists is used to highlight that sustainability is made of different coexistent realities that cannot be reduced to a single existing paradigm. The then research proposes a 2-phases and 2-domains for regional sustainability which aims to model complex regional sustainability. The possible application of this framework in a variety of projects types is presented. This research proposes new models for sustainability that are more inclusive, and that are rooted in the complexity of regional objectivity and regional subjectivity. The paper specifically contributes to the theory of sustainable design by clarifying the unique and non-conflicting sustainability expertise that co-exist at the local/regional and expertise domains.