The performance gap between the design energy consumption of buildings and their real energy consumption has three main sources: the energy systems' performance, the users' behaviour and the buildings' envelope performance. The latter should be characterized under in-use conditions by estimating their in-use Heat Loss Coefficient (HLC). This work further develops an existing 'average method' by fully developing it from the energy conservation principle applied to a generic in-use building. Furthermore, the uncertainty sources are identified and limited through the mathematical development of the method. An innovative solution to the problematic of multizone buildings is also demonstrated, where HLC values should be calculated for different floors and then aggregated to obtain the entire building's HLC. Furthermore, all these can be done without the need of a detailed model of the building. The improved average method has been applied to an occupied, energetically monitored office building of the University of the Basque Country. The building was energetically rehabilitated during the summer of 2017. Therefore, the proposed method has been applied over the three winters prior to rehabilitation and then, to the winter after the rehabilitation. It has thus been possible to estimate a 28% reduction of the HLC for the post-retrofitted case, as compared to the pre-retrofitted one. (C) 2019 Elsevier B.V. All rights reserved.