Although the massive overuse of ecological resources still barely carries any financial weight, this overshoot cannot persist. The consequence of continuing down the path of depleting resource stocks and overloading waste sinks (such as the atmosphere with GHG) is that markets will not adjust smoothly, but rather be faced with disruption. But are we resigned to one-planet misery? Or can we choose to design our path to one-planet prosperity, where all thrive within the ecological budget of our planet? More than half of humanity already lives in cities. The built environment, including residential, commercial and industrial buildings, as well as roads and energy infrastructure, is an especially powerful lever since its legacy endures for decades, even centuries. Retrofitting old structures into beacons of sustainable living is an especially meaningful and effective way to transform our cities so that humanity can reach and sustain one-planet prosperity.