Over the last two decades, there has been considerable progress in developing solutions (mostly technological) for the design and construction of sustainable, low-carbon architecture and built environment. More recently, there has been success (albeit limited) in designing/researching architecture that may promote better human health and improve wellbeing. What is less clear is how these two disciplines (sustainability and health) might better overlap if we are to address the two grand challenges facing society: combatting climate change and improving human health. This research is urgently required to address these issues, particularly as cities are now one of the largest contributors to the climate crisis. The design of the built environment plays a significant part in both of these challenges and any solution requires integrated and holistic resolution. The research looks at the role ‘codesign’ might play in unifying these two fields. One of the criticisms of solutions developed thus far is that they tend to be overly technological, often at the cost of elaborating more sociological, economic, behavioural (and hybrids thereof) alternatives. Codesign involves greater democracy in the design process and brings together social, technological and political realms. Codesign in this context often leads to synthesis of traditional knowledge and practices; particularly when combined with nature-based solutions. The research concludes that a triple shift is required: from centralised to messy-networks; from fossil-fuel to nature-based; and from technical ‘solutions’ to codesigned alternatives.