Biomass-based district heating networks and systems play a central role in sustainable heat supply covering around 2,400 systems in operation in Austria. In order to meet current and future technical, economic and regulatory challenges combined with a sustainable and strategic expansion of the heating system, there is an increased need for retrofitting and modernization of the existing heating networks, especially of the first and second generation. BM Retrofit aims to develop and demonstrate holistic retrofitting and modernization concepts as well as portfolio expansion of biomass-based district heating networks and systems. Within the project, innovative technical concepts (e. g. flue gas condensation plants, heat pump systems, storage technologies) will be developed accordingly and optimized for efficient system integration (Technology Readiness Level TRL: 6-8). Two to three demonstrator sites will be used to showcase, validate and optimize the developed retrofitting concepts. The results and experiences from the planning, implementation and operation phase to derive guidelines including lessons learnt and best practices, highlighting promising concepts and pathways. The proposed holistic systemic approach, consisting of technical, non-technical and systemic measures, results in high synergy potentials to a) adapt and further develop existing district heating networks to meet future requirements, b) achieve corresponding climate goals and c) strengthen economic benefits including local value chain creation. The EEG is responsible for evaluating the project results in terms of achieving the GEL objectives, defining scaling scenarios and evaluating market potential, determining the impacts of the implemented retrofit measures on the energy system.