The earthquake in Zagreb on March 22, 2020 damaged many buildings, most of which were masonry and constructed before the use of earthquake-resistant construction regulations. That quake indicated the need for strengthening masonry buildings for seismic action. Seismic forces are mostly carried by the in-plane shear walls. Therefore, the main problem is proper calculation of the required strengthening of the in-plane shear walls and solving the strengthening details, but not only locally, at the level of individual walls, but also at the level of the whole building, to properly cover all mechanisms of masonry failure and transferring seismic forces to the foundation soil. The paper presents the most common methods of calculation of in-plane shear wall strengthening with a review of details that are not sufficiently well clarified or are neglected, i.e., poorly used in practice. In addition to the calculation of strengthening of masonry with the reinforced concrete layer (shotcrete) and dowels, modern methods of strengthening are considered, using FRP fabrics or plates and FRCM systems. As these calculation methods are not implemented in the existing codes, various recommendations are used based on similar assumptions, but with different input parameters and safety factors that affect the calculation result and that should be properly interpreted and used in the calculation.