The present paper deals with the case study of structural strengthening of a landmark building complex in the center of Athens. As part of its ongoing complete rehabilitation, the seismic strengthening of the building was considered in order to increase its seismic safety and prolong its life span. The paper concentrates on the central building which has a height of 65m and consists of three structural independent units. The strengthening strategy adopted includes the use of reinforced concrete jackets to improve the stiffness, strength and ductility of the building. The paper presents in detail the steps followed for the structural design. First the investigations performed for the documentation of the structural system are presented. In the sequel, the major strengthening measures are presented. The analysis procedure is presented and the most significant results. After strengthening, the building meets the requirements of Eurocode 8 and the relevant Greek Code of seismic retrofitting of existing buildings.