Software Defined Buildings (SDB) seeks to design, engineer, and evaluate the foundational information substrate for cyberphysical systems in a concrete, canonical form - creation of efficient, agile, model-driven, human-centered building systems. Modern commercial buildings provide increasingly integrated Building Management Systems, but are typically closed or based on proprietary interfaces, are difficult to extend, and it is expensive to add new capabilities. Key underlying technology trends driving building system design are the rapidly declining costs of new cyber technologies, e.g., ubiquitous wireless communications, mobile devices, cheap embedded processing, and scalable processing and storage in the Cloud. The research challenge is how to extend the building “machine,” to harness these trends, making it better, more capable, and more efficient. Our approach is to develop software-defined buildings, to shatter existing stovepipe architectures, dramatically reduce the effort to add new functions and applications without “forklift upgrades,” and expand communications and control capabilities beyond a single stand-alone building to enable groups of buildings to behave cooperatively and in cooperation with the energy grid.