Apparatus, systems and methods for ascertaining the occupancy of a building are presented. The building is divided into one or more control zones which correspond to physical areas of the building associated with controllable modules, such as HVAC units, lighting, irrigation, or other environmental features such as fountains, music, video, and the like. Zone parameters define how zone devices shall react to the number of occupants located in the particular zone. A building control system detects individual mobile devices in and around the building, and determines the locations of each device by using trilateration and/or location services. The identified mobile devices act as proxies for building occupants. The locations of these devices are correlated with the locations of the zones in the building, and the building control system then adjusts the operating parameters of the zone based on the number of devices present in the zone.