This paper presents aspects of an office renovation project of the Social Insurance Institute (SII), the largest Social Security Organization in Greece. The project refers to small and larger SII office buildings, including various stages of intervention, ranging from the construction of new buildings, to renovation of existing buildings. Construction and electro-mechanical studies for each building were carried out in order to implement the appropriate and feasible actions. Mechanical ventilation was implemented in all spaces, including forced air circulation through wall or ceiling-mounted air ducts and hybrid ventilation. Energy saving strategies were also implemented including shading, solar-absorbent glazing, night ventilation, energy efficient lamps and BMS controlled A/C system. Two case-studies of buildings in the area of Northern Greece are briefly presented. Thermal comfort conditions and energy consumptions are examined for various retrofitting scenarios and results are discussed.