This paper presents the application of the jetgrouting method as a structural and ground improvement technique for strengthening the soil-raft foundation system of an existing reinforced concrete building. The structural system of the originally designed superstructure consists of columns for bearing the vertical loads and shear walls for ensuring the adequate seismic response. The building has been executed up until level zero by 2010. However, during construction, the investment intensions have been changed and the owner has decided to extend the structure by additional floors which in turn has caused the need of redesign of the building above the ground level and strengthening of its underground part. The aim of the study is to demonstrate the adopted design approach for strengthening the soil-raft foundation system and applied methodology for proving the predicted jet-grouting properties (diameter, length, compressive strength and elasticity modulus) as well as to outline the difficulties which have occurred during execution and the solutions of some important problems.