Ga.o House is a tropical live–work prototype in Da Nang, Vietnam, built on a 111 m² urban plot with a total built area of 172 m². Organized as a 50/50 residence–workplace model and completed with a modest budget of approximately USD 130,000, it responds to the context of dense tropical urbanization, limited land, and the growing overlap between domestic and professional life. The design challenge was to create a compact house that could reduce environmental impact without relying on expensive or hidden technical solutions. Instead, sustainability is treated as an architectural system. A prefabricated bolted steel frame reduces reliance on concrete and allows future disassembly, while reclaimed wood and repurposed excavation stone extend the life of existing materials. Shaded openings, deep balconies, cross-ventilation, daylight optimization, and a vertical green void improve comfort while reducing dependence on mechanical systems. Solar PV generates approximately 11 MWh/year, while rainwater harvesting collects around 130–180 m³/year for reuse, making environmental performance visible and understandable in daily life. As an educational device, Ga.o House shows how construction logic, material reuse, climate response, energy production, water cycles, cost awareness, and daily habits can work together as one low-impact living system. It demonstrates that sustainable architecture can begin with ordinary, precise, and responsible decisions.