Within the framework of a refurbishment project of a historical unreinforced masonry (URM) building with wood beam floors in Zurich, Switzerland, the structural seismic safety was assessed by a third party. A conventional, force-based approach was used in the assessment, which did not appropriately consider the seismic behaviour of the wood beam floors and the URM walls. Consequently, a new construction of stiffening steel trusses on new pile foundations was recommended as retrofit measure, greatly exceeding the planned budget for seismic measures. Within the framework of a second opinion, we firstly assessed relevant structural details in situ. Secondly, the mechanical properties of URM walls were measured via rebound hammers and flat-jacks. Lastly, we carried out a displacement-based seismic analysis of the structure, as well as stability analyses of façade walls. This way, all of the previously proposed seismic retrofit measures were shown to be unnecessary. This example shows the necessity of thorough structural assessment and detailed in-plane and out-of plane earthquake analyses, in order to correctly assess the seismic behaviour of historical URM buildings without floor diaphragms. With this approach, the retrofit costs can often be vastly reduced in comparison to a conventional, force-based seismic retrofit. This example highlights the advantage of commissioning highly experienced seismic engineers with the seismic assessment of historical structures.