The expandable house (‘rumah tambah’ in Bahasa Indonesia, or rubah for short) is designed to be one part of a sustainable response to the challenges of rapidly developing cities like Batam, in Indonesia’s Riau Archipelago. Once a collection of sleepy fishing villages of a few thousand inhabitants, Batam developed to be a cosmopolitan city of over one million people in less than 40 years. This remarkable growth, fueled by a new free trade agreement and Batam’s proximity to Singapore, has not abated. Young migrants from all over Indonesia moved there to seek their fortune. By 2015 Batam was named the fastest growing city in the world. As a consequence, the city was confronted by major planning problems: How to accommodate the influx of migrants? How to provide sufficient housing, and appropriate water and sewage systems? How to fund adequate transport infrastructure, and provide schools, universities and hospitals?