Within this work, life cycle assessment modeling is used to determine top design priorities and quantitatively inform sustainable design decision-making for a prefabricated modular building. A case-study life-cycle assessment was performed for a 5,000 ft 2 prefabricated commercial building constructed in San Francisco, California, and scenario analysis was run examining the life cycle environmental impacts of various energy and material design substitutions, and a structural design change. Results show that even for a highly energy-efficient modular building, the top design priority is still minimizing operational energy impacts, since this strongly dominates the building life cycle's environmental impacts. However, as an energy-efficient building approaches net zero energy, manufacturing-phase impacts are dominant, and a new set of design priorities emerges. Transportation and end-of-life disposal impacts were of low to negligible importance in both cases.