To implement the UN Sustainable Development Goals (SDGs) and sustainability more broadly, the built environment must not only reduce harm but actively reverse ecological decline, planetary overshoot, and structural inequities. Given their massive lifecycle impacts, buildings and urban development must be radically reconceived and retrofitted. This research applies Positive Development (PD) theory, which reframes sustainability by showing how cities and buildings can expand global ecological and social capacity. PD contends that genuine sustainability requires all structures to be net-positive, meaning they must expand the ‘ecological base’ (increasing global nature, not merely restoring it) and the ‘public estate’ (increasing universal environmental and social justice). The initial research question was whether net-positive sustainability could be achieved at the global scale using existing sustainability frameworks. The key finding was that new standards, methods, and metrics are essential. To meet this challenge, the STARfish net-positive design app was developed. It is an open-source design and assessment tool that operationalizes PD principles. STARfish can transforms the 17 SDGs into a net-positive framework by establishing absolute sustainability baselines, rather than relying on incremental gains over existing conditions. Its dynamic fractal diagram makes cumulative ecological and social gains or losses visible beyond project boundaries, enabling designers to estimate impacts in real time. Because it can assess multifunctional design, STARfish enables projects to deliver more public benefits per unit of resource than efficiency-based approaches. STARfish is adaptable to local and regional contexts while maintaining universal standards, so it is easily transferable to developing nations. Hence, it provides a practical pathway for embedding net-positive design into global practice. This paper focuses on how the UN SDGs can be actualized through net-positive design. The potential impact of STARfish ultimately depends on its uptake.