Refurbishment of old existing buildings means an enormous potential for improving their energy efficiency. Besides the established renovation methods encompassing mostly measures applied to the thermal envelope, the extension of existing buildings with lightweight structural upgrades has become popular especially in densely populated urban areas. While being beneficial to the reduction of the transmission losses through the roof of the existing building, the above solution also results in acquisition of new floor surface. Despite the increasing number of such renovations in practice, this topic is rather poorly supported by scientific research, which results in a lack of general design guidelines. The paper discusses the impact the timber-glass upgrade module has on energy-efficient refurbishment of multi-family buildings. The main variable design parameters of the numerical study are the height of the upgrade module and the existing building, relevant to the number of storeys, alongside with the thermal transmittance of the module envelope elements, its glazing size and orientation. The key of the study is to explore to which extent the application of the timber-glass upgrade module influences the energy need of the hybrid building regarding the aforementioned design parameters with the aim of establishing design guidelines for refurbishment with attic extension. (C) 2018 Elsevier Ltd. All rights reserved.