The demand for participant engagement in urban planning shows a great need for tools that enable communication between stakeholders and make planning processes more transparent. So far, common methods use different tools and platforms independently. This prevents the full potential for effective, efficient, and creative collaboration from being realized. Hence, this paper presents an approach that combines different participation settings (off-site, on-site, and online) by using an interactive touch table and an additional screen, as well as virtual reality (VR) and augmented reality (AR) devices, and synchronizing them in real-time. To fulfill the collaboration requirements, three major technical aspects are addressed in the concept and prototype implementation: Firstly, the demands for various settings and devices require a uniform and cross-device interaction concept. Secondly, all changes in the course of the participation (e.g., adding, manipulating, or removing objects) must be synchronized across all devices in real-time, with very low latency. Thirdly, the various states should be saved persistently during the collaboration process. Detailed empirical usability studies are still pending; however, pretests indicate that the concept is appreciated, and the transferability to other planning processes is given.