The ambitious target of the European roadmap for the 2050 low-carbon economy is to reduce the overall equivalent CO2 emissions by 2050 to below 80% compared to the 1990 level in a cost-efficient way. These targets urge a significant transformation of the EU energy systems from the traditional fossil-based systems towards high-efficient renewable based energy systems. The main objective of this project is to systematically understand and propose the techno-economic-optimal transformation pathways for the Finnish building stocks, national energy systems, and grids to progress towards the EU 2050 energy/emission target. From building and building stock sides, the transformation paths for the newly established and retrofitted existing building stocks. From the national energy system side, the objective is to analyze the energy system in achieving the future energy and emission targets. The specific objective for the grids is to support renewable generation sources.