The challenges of the modern world, such as urban population growth and climate change, have necessitated a change in cities. Current trends related to the design of cities have highlight the essence of mixed-use urban areas. Determining mixed-use at a fine-grained level is challenging. Our new approach uses the ‘floating window’ method to determine the fine-grained local values of mixed-use indicators based on data from multiple sources. This method captures a continuous picture of changes in the values of indicators across the city. To test the developed analysis methodology, we performed a case study for the city of Piaseczno, located 18 km from Warsaw, the capital city of Poland. We have shown that mixed-use indicators defined in various ways can lead to varying assessments across the city. To solve this problem, we proposed a combined mixed-use indicator. This method is an innovative approach to analyzing a data set, because it is independent of imposed administrative boundaries or other city divisions into smaller areas. The developed methodology allows for a diagnosis of cities and, by introducing measurable indicators, selecting areas for improving the quality of life is improved over methods based only on the available statistical data.