Buildings account for more than 40% of Europe’s Energy consumption, where about 64% is accounted for by existing buildings. 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.The methods to identify the most cost-effective retrofit measures for particular buildings are still a major technical challenge, which is leading to a “progressive widening of the gap between theory and practice”. Drawing on the lack of real life data for whole buildings and technologies energy use patterns, a baseline of existing whole buildings and technologies energy usage (pre-retrofitting) must be established from which improvement and efficiencies can be measured. Since, one can't simply compare the total amount of energy the building used in the year before and the year after, due to weather difference conditions from year to year, so an abnormally warm winter the year before followed by a really cold winter might cause the house to use even more energy than before. This paper recommends a systematic approach to proper selection of energy monitoring and explores a weather-normalisation method for estimating the energy savings due to building retrofit. The energy signature and the so-called Normalised-Annual-Consumption were developed from the utilities (gas and electrical) meter readings/bills. The paper also describes the propose methodology for an on-going longitudinal energy monitoring (post-retrofitting) to determine actual energy saving brought about by a wide range of retrofitting technologies.