This paper addresses the technical, financial, and policy challenges in achieving substantial decarbonization of existing large commercial buildings operating in the northeast. Decarbonizing existing large buildings is particularly important since it can have a large and fast reduction of a building portfolio’s emissions. However existing large buildings have special considerations due to the need to meet the building constraints of their existing HVAC and electrical systems. Consideration of a hybrid strategy, is presented, utilizing both electric heat pumps and conventional natural gas boilers already installed.