In Europe, energy consumption by the building sector accounts for more than 40% and emits about 36% of CO2 emissions, and it has been increasing in recent decades. Consequently, EU policymakers have long recognised the importance of energy-efficient buildings in mitigating climate change and have recently issued the 2012 Energy Efficiency Directive (EED), but capturing that potential has posed a challenge. While the efficiency of new buildings has improved over time, most of Europe’s existing building stock – over 90% of the total – has yet to be affected by energy performance requirements. Retrofitting of these existing buildings offers significant opportunities for achieving the EU objective of reducing energy consumption and greenhouse gas emissions by 20% by 2020. Each existing building, its systems and occupants are unique and connected; accordingly, there is not one retrofit method that will work for all existing buildings. Additionally, identifying the most cost-effective retrofit measure for particular building components is still a major technical challenge which is leading to a “progressive widening of the gap between theory and practice”. Holistic energy retrofit strategies, in which all the interactions in a building’s envelopes, systems, controls, occupants behaviour, cost-optimization and energy-efficiency goals have been proposed to yield substantially higher energy savings of about 80%. This paper presents experiences of holistic energy-efficient retrofitting of 1920s houses in the UK with highly technologically advanced renovation methods, which have been investigated and analysed at the demonstration projects realised in the FP7 project HERB (Holistic Energy-efficient Retrofitting of residential Buildings) (http://www.euroretrofit.com).