We calculate the benefits and costs of implementing the International Association of Plumbing and Mechanical Officials (IAPMO) 2012 Green Plumbing and Mechanical Code Supplement (GPMC) for various building types in Hawaii, with particular emphasis on water-use efficiency provisions in the code. Benefits of the GPMC are measured as water savings, where baseline usage is estimated in accordance with the 2012 Uniform Plumbing Code (UPC), which has been recently adopted by the state and will soon be adopted by the counties. We also monetize those benefits at the household level (water bill savings) and at the state level (cost savings to the water supply boards and departments throughout the state). Based on discussions with plumbers, building contractors, developers, architects, mechanical engineers, planners, and other water specialists, as well as an assessment of prices at major home improvement stores and other online retailers, we estimate the costs of GPMC compliance for new structures planned for Hawaii over the next decade. If the GPMC is implemented, the payback period is two years and the net present value assuming a discount rate of zero is $15.13 million. For a discount rate of 5%, the NPV is $11.29 million.