The aim of this paper was to develop a social sustainability tool to clarify what social sustainability means in land use planning processes. The tool was developed through a robust research process covering a large amount of scientific literature and built environment sustainability assessment tools. To assess how the developed tool works in practice, we used it as a coding system for the assessment of social sustainability in a case study from Finland. The case study includes the master planning process of a medium-sized city and the legal framework guiding it. The theoretical contribution of the paper lies in creating a holistic tool for the assessment of social sustainability in planning processes. This study also provides a strong practical contribution, as it goes beyond theoretical definitions to the pragmatic level by providing exemplary measures to be used in designing and implementing processes. Therefore, this paper is a valuable source for those stakeholders (for example planners, developers and policymakers), who struggle with the issue of the successful implementation of social sustainability practices, and it will help them to benchmark their processes against the themes found in the tool.