The aim of this study is to characterize the energy performance gap in retrofit of multi-family residential buildings, and to assess its potential impact on the energy savings of the Geneva post-war building stock. In a first step, we analyse 10 recently retrofitted case studies, covering around 1'100 flats. For each retrofit, the theoretical and actual energy savings for space heating are calculated on the basis of: (i) measured final energy demand for heating (SH and DHW), before and after retrofit; (ii) design value for SH after retrofit, according to the SIA 380/1. As a major result of the study, a robust statistical correlation between theoretical and actual energy savings allows to characterize the energy performance gap. In a second step, this result is used to assess a realistic energy saving potential for Geneva's multifamily building stock. This assessment shows that, under current practice, only 42% of the theoretical energy saving potential of building retrofit could be achieved. Finally, the main reasons behind this gap are discussed, as well as its potential effect on the goals of the Energy Strategy 2050.