As the climate crisis accelerates, policymakers are increasingly turning to novel forms of regulation to reduce greenhouse gas (GHG) emissions. These include regulating the emissions of buildings, which account for over 30 percent of GHG emissions nationwide. A key issue in regulating buildings is whether to regulate building GHG emissions, or whether to regulate the energy efficiency of buildings. The metric for regulation is important because it will determine what behavioral changes the regulation induces. This essay argues for regulating building energy efficiency. In doing so, the essay draws on a pioneering study, published in 2021, examining a landmark New York City law that regulates building GHG emissions. The study’s analysis of the expected impacts of the city law suggests the perils of regulating GHG emissions from buildings, which are likely to be outside of the control of the regulating jurisdiction.