RetroKit will develop and demonstrate multifunctional, modular, low cost and easy to install prefabricated modules, integrating efficient energy use systems and RES for systemic retrofitting of residential buildings. Aiming at external retrofitting concepts with an integration of internal renovation aspects, semi-prefabricated technological solutions will be clustered in a “RetroKit Toolbox” supported by a decision support system that provides data, transparency on payback, guidelines, and design support. Solutions will target aging multi-family residential buildings that represent more than half of the European building stock and consume between 65% and 80% of the total energy consumptions of EU buildings.RetroKit targets the social, technological, industrial, and economic barriers preventing retrofit. RetroKit overcomes these barriers by addressing the highest potential energy savings strategies, developing configurations of semi-prefabricated modules for industrial scale-up that integrate different functionalities into façades and roofs. RetroKit solutions include RES, free cooling and solar heating strategies, while LCA, LCC and business models will support the market take-up. RetroKit systemic approach to retrofitting will streamline the value chain, thus enabling customised mass production, industrial scale-up, and efficient construction processes for manufacturers, integrators, installers and contractors.RetroKit workplan align the S&T Objectives, with the methodology and project impact. Clear, quantified, and measurable performance metrics are established. Cost estimates for retrofit performances are already conducted demonstrating that a ROI close to 7 years is achievable by RetroKit solutions. Testing will make use of partner world class laboratories and building test facilities. Validation will involve 3 full scale buildings in different climate zones and apply the international standard IPMVP for energy savings assessment.