The project aims at bridging the knowledge gap in building retrofitting. The existing supporting data for energy efficiency has many shortcomings which have lead to many blunt efficiency plans in cities and large building owners. KTH has for several years conducted research on urban metabolism. A key part of this work consists of big data analysis of energy use in buildings. The research has access to a unique data set consisting of 130 million lines of district data for 15,000 properties, 58,000 energy assessments, as well as 3D models of the whole city. The goal of the project is to develop a replicable methodology for developing energy efficiency plans that indicate each building economically sustainable efficiency potential and its payback time. Based on this method, a plan will be developed for the City of Stockholm, and one plan for “Sjöstadsföreningen i Hammarby Sjöstad”. In the following project phase, these plans will be evaluated.