In the last decade, maintenance and adaptive reuse, of the existing building stock, have gained ground in the construction sector. For the adaptive reuse of a building, it is required to analyze, meticulously, the existing structure. Unreinforced masonry structures are indicative of the Dutch built environment. For the assessment of their structural capacity, the European Standard EN 1996 is, currently, applied. Masonry walls of high slenderness, often, comprise the structure of existing buildings, in the Netherlands. It has been noticed that the EN 1996 norm underestimates the vertical resistance of slender walls. This study attempts to extend the applicability of the EN 1996 norm to slender masonry walls, since the norm serves for the assessment of existing structures. Particularly, the research objective is to find an appropriate verification method for existing slender masonry walls, in one-way bending. Literature research is, initially, conducted, with respect to alternative formulas for the calculation of the vertical resistance of slender masonry walls. The formula in the EN 1996 norm and the alternative formulas are reviewed, considering a case study. The case study is one of the slenderest interior masonry walls, that form the structure of a relevant existing building block in Amsterdam. The latter was constructed in the late 19th – early 20th century. The engineering firm STRACKEE BV Bouwadviesbureau provided the technical drawings of the building block. Reference values for the vertical resistance, of existing slender masonry walls, are necessary to develop an appropriate formula for their verification. Therefore, the next step of the research is the numerical analysis. The response of slender masonry walls, subjected to combined vertical and lateral loading, is estimated according to the results of FE analysis. The case study is the reference for the geometry of the FE model. Indicative, for the construction period, properties of masonry are the input for the initial material model. Further, models of slender masonry walls with different geometrical and material properties are analyzed. Specifically, a parametric study is done, to define the influence of geometrical and material properties on the vertical resistance. Based on the FE analysis results, a new formula is proposed. The formula estimates the vertical resistance of existing slender masonry walls, subjected to combined vertical and lateral loading.