Improving the efficiency of existing buildings and eliminating fossil fuel usage are vital steps to meeting New England state greenhouse gas emission reduction goals. This session focuses on the best ideas and lessons learned from MassCEC’s Triple Decker Design Challenge. What are the most important parts of a triple decker to focus on if you have limited funds? Will we need to panelize as per the Energie Sprong model to reasonably electrify this scale of building? How important is reduction of air infiltration to successful decarbonization? How does solar PV change 30-year operating costs and are there ways to get over landlord/tenant split incentives for solar PV? MassCEC will answer these questions and more by summarizing major conclusions from review of the 13 proposals in the January 2021 competition. Two winning applicants, one with a lower cost retrofit proposal and one with a proposal that adds a unit and retrofits the existing building will describe decisions and challenges they faced. The session will end with a discussion of how policy and incentives can support decarbonization of existing triple deckers.