This 2019 paper from the Emmett Environmental Law & Policy Clinic at Harvard examines legal complexities that Massachusetts towns and cities must navigate when mandating reductions of greenhouse gas emissions from the building sector. It focuses on both new and existing buildings. Successful mandatory approaches to net zero buildings must fit into the framework of federal and state laws so as to avoid being preempted and should weave together community-level rules and policies to support successful implementation with an eye to impacts and equity. This paper evaluates a range of municipal options, concludes that several are viable with existing authority, and then provides an annotated model ordinance for one. The paper also briefly outlines ideas for voluntary and incentive-based approaches to promoting net zero building goals.