Urban planning requires collaboration of multiple stakeholders exploring a diverse set of design options in real time charrettes. Multi-touch tabletop applications can support collaboration with custom-developed tools, but evolving and scaling these systems is challenging. By leveraging cloud-based architectures and powerful gaming engines, we can allow urban design scenarios to become fully immersive and interactive in Augmented and Virtual Reality (AR/VR) environments. Our work explores architectural tradeoffs in this domain.