Following the need of urban areas to maintain the existing building stock and simultaneously upgrade the overall energy performance, the renovation down-to-nZEB state has already become a necessity. In this regard, a vast range of prefabricated solutions have been developed lately. The main objective of such solutions would be not only to constitute an effective system to tackle building energy consumption but also to be versatile in terms of implementation and economic viability. In this regard, an adaptable off-site prefabricated envelope solution with an embodied HVAC system called “SmartWall” has been developed. The SmartWall can minimise thermal losses through the well-insulated envelope while, at the same time, its integrated HVAC system efficiently maintains indoor thermal comfort conditions. This study examines the virtual implementation of the SmartWall as a “Plug-n-Play” renovation solution to reach the nZEB state of a typical apartment in a multi-family residence in Athens. The analysis considers two SmartWall alternatives using conventional and eco-friendly materials. The results indicate a reduction of 88% in primary energy consumption without affecting thermal comfort conditions and highlighting that the nZEB state can be ensured if the SmartWall application is enhanced with photovoltaic modules.