A ground motion with a PGA of 0.39 g, above the 475-year design earthquake specified in the Greek seismic code and the National Annex to Eurocode 8, was recorded about 100 m from the municipal theatre of Argostoli, the capital of the Ionian island of Kefalonia. The building, the largest in volume in the town, was designed and constructed to the 1959 seismic code for lateral loads (about 30 % of those specified today). Suffering also from severe reinforcement corrosion and poor concrete quality, it provided the ground to apply for the first time (in 2005) Part 3 of Eurocode 8 for seismic assessment and retrofitting. The assessment based on nonlinear dynamic analysis showed that the buidling was deficient even under ground motions with PGA less than 0.05 g. Alongside the need to tackle corrosion, the design of the retrofitting had to face several constraints and requirements: minimal disruption of the facility’s operation, accessibility to the foundation only along the building perimeter, no change to the façade and minimal intervention to the other lateral sides. A very cost-effective retrofitting was designed, employing: (a) one-sided RC or fibre-reinforced polymer (FRP) overlays and two new walls at the perimeter, (b) connection of the two structurally independent and torsionally imbalanced units of the building in a single structure, and (c) one-sided FRP jacketing of selected interior elements. Less than 7 years after the works were completed, the building went unscathed through a Magnitude 6.1 earthquake. Serious damage was limited to four nonstructural masonry infill panels at the penthouse, which, according to the retrofit design, could be sacrificed, so as to protect vulnerable structural elements that could not be retrofitted in the first place. Nonlinear dynamic analyses of the response of the retrofitted building to the recorded ground motions are in agreement with the observed performance. They show also that, had the as-built facility not been retrofitted, it might have collapsed in the earthquake.