Buildings are arguably more meaningfully approached as process versus product. We cannot decarbonize a part of this process (e.g., operation stage) and thereby claim such building(s) as ‘sustainable.’ A zero-carbon structure with no robust functional flexibility will become obsolete long before its physical life concludes. The present research reveals that robust sustainability resides at the nexus of durability, flexibility, and responsibility principles. The author suggests that such principles are not mutually exclusive nor incompatible with values of aesthetics and performance. Through the study of flexible architecture, particularly in residential projects, the present thesis establishes a novel multi-criteria decision-making framework for design projects. The theoretical framework, offering holistic and unified design criteria, corresponds to physical, functional, economic, technological, social, legal, and political facets that instigate and propel building. The methodological approach of the research follows three sequential stages: literature meta-analysis, a survey of experts, and case studies. Theory is evoked, including recent considerations of open building, holistic design, and systems thinking (Langston 2013, Sinclair 2009, 2015, 2016; Imam and Sinclair, 2018, 2020, 2021). Industry perceptions regarding environmental, social, and economic tenets of sustainable development are identified via a purposive survey with 69 architects and researchers. Seminal cases of eight (8) award-winning projects are drawn from regions with the highest reported current and projected floor area, illustrating agility concepts in design, legislative, and/or financial ethos. Case study data, together with the strategic literature review and survey, highlights leading themes, suggesting agile systems that are composed of four key characteristics: 1) diverse approaches and strategies that can learn (design intelligence), 2) the relationship among systems and sub-systems is nonlinear, 3) co-evolve with their environment, and 4) display emergent properties. The large-scale impact of climate change that is accelerating before our eyes, now complicated by the pandemic, calls us to be resilient and more open-ended in designing our built environment. Agility proves a compelling means to proffer more inclusive distributed decision-making structures appropriate for daunting and rapidly evolving realities of contemporary life. Agility is not optional - rather, it is essential to next iterations of responsive & responsible process, product, space, and place.