Energy use in buildings accounts for approximately 40% of EU energy consumption. Energy efficiency in new buildings is important, but existing building stock is the main target. Existing buildings, however, are characterised by particular requirements and constraints that are not present in new buildings and that requires new developments and adaptation of existing technologies.In order to fulfil the most recent EU directives, solutions for a drastic reduction in primary energy consumption are required. Space heating and domestic hot water (DHW) represent the largest part of energy use in buildings nowadays, thus solar thermal energy seems to be one of the most promising heat source.The overall objective of EINSTEIN project is the development, evaluation and demonstration of a low energy heating system based on Seasonal Thermal Energy Storage (STES) concept in combination with heat pumps for space heating and DHW requirements for existing buildings to drastically reduce energy consumption (primary energy savings up to 70% compared to conventional existing thermal systems).This goal will be achieved by:- Technological developments for STES systems adaptation for existing buildings and integration with the built environment- Development of a novel, high-efficiency, cost-effective and compact heat pump suitable for existing buildings and optimized for higher temperature heat sources such as STES systems- Development of new business and cost models which consider the entire life cycle of a building and incorporate the benefits of reduced operating costs; a decision support tool will help the planners to find the best technology to install in each particular case- Development of integrated building concept. As cost-effectiveness is one of the main aspects to be considered in building retrofitting, a methodology and a software tool for most cost-effective global energy intervention framework definition for building retrofitting will be developed