This paper presents a visibility analysis tool for 3D urban environments and its possible applications for urban design practice. Literature exists for performing visibility analysis using various methods and techniques, however, tools that result from such research are generally not suitable for use by designers in practice. Our visibility analysis tool resides in Grasshopper, Rhino. It uses a ray casting method to analyze the visibility of façade surfaces from a given vantage point, and of a given urban setting, in particular, buildings and roads. The latter analysis provides information on the best visible buildings/building facades from segments of roads. We established a collaboration with a practicing architect to work on a design competition together, using this tool. The paper elaborates on the visibility analysis methods, presents the tool in detail, discusses the results of our joint work on the competition, and briefly reflects on the evaluation of the use of the tool by design practitioners.