Map-based web applications are often used to display spatial information in the context of (open) data from the environmental and urban infrastructure sectors. As the number of thematic layers displayed on these applications grows, maintaining user interface intuitiveness becomes increasingly challenging. To provide an initial account of design practices in the area, this article examines 19 map-based applications with multiple thematic layers, along four dimensions: thematic content complexity, functionalities for layer search, legend design and functionalities for the search of location information. We then report on findings from a brainwriting workshop involving 11 participants, during which three applications were used to identify design challenges and opportunities. The observations and design issues presented are relevant to researchers and designers of applications displaying multi-theme spatial information.