Smart Built Environment is an eco-system of `connected' and `smart' Internet of Things (IoT) devices that are embedded in a built environment. Smart lighting is an important category of smart IoT devices that has recently attracted research interest, particularly for residential areas. In this paper, we present an extended reality based smart lighting design testbed that can generate design prototypes based on the functionality of the physical environment. The emphasis is on designing a smart lighting system in a controlled residential environment, with some evaluation of well-being and comfort.