Many reinforced concrete (RC) structures, built in seismic-prone countries before the introduction of the modern seismic oriented codes and usually designed for gravity loads only, necessitate an upgrade in terms of strength and ductility against lateral loading. In this paper the possibility of using post-installed anchors for seismic retrofit solutions is investigated. Post-installed anchors are usually fast and easy to install and they represent a valuable low-invasive solution to transfer high loads with quite low costs. The retrofit of RC beam-column connections with a diagonal haunch element fastened to the existing structural element using post-installed anchors is proposed. The design method based on experimental and analytical investigations is presented. Particular focus is given to the requirements in terms of load displacement characteristics, i.e. stiffness on the post-installed anchors.