In the thesis, we review the method for determining the mapping of vacant land (as specified in Regulation on determining land for development, Official Gazette of R. Slovenia, no. 66/13) created with the purpose of imposing tax on such land. We focus on criteria applied by the method. We assume these criteria can never account for all practical spatial situations, this resulting in differing interpretations of the same criterion. While this may hardly be surprising from the point of view of spatial planning, it becomes controversial when viewed upon from the perspective of equality and tax fairness. We conclude that the selection of a land policy instrument can only be a direct consequence of a recognized spatial problem. We examine international practice, test the mapping method on four Slovenian municipalities (Ljubljana, Kranj, Divača, Kanal ob Soči) and conduct a survey among all Slovenian municipalities. Targeting a better activation of catalogued land and determining its true condition and potential, we use our results to conclude with a suggestion for establishing a public vacant land catalogue in form of an interactive web portal, offering users the information on spatial situation and trends while allowing them to add newly recognized vacant or degraded lands and submit assessments, comments, suggestions or initiatives. The inclusion of local inhabitants and land owners in such or similar manner can contribute significantly to general awareness of spatial planning issues and the importance of sustainable development as well as to the accuracy and completeness of the vacant land catalogue, which can ultimately improve legitimacy and feasibility of spatial planning decisions.