We present a digital-physical system to support human-computer collaborative design. The system consists of a sensor-instrumented “sand table” functioning as a tangible space for exploring early-stage design decisions. Using our system, human designers generate physical representations of design solutions, while monitoring a visualization of the solutions’ objective space. Concurrently, an AI system uses the vicinity of the human’s exploration point to continuously seed its search and suggest design alternatives. We present an experimental study comparing this side-by-side design space exploration to human-only design exploration and to AI-only optimization. We find that side-by-side collaboration of a human and a computer significantly improves design outcomes and offers benefits in terms of the user experience. However, side-by-side human-computer design also leads to more narrow design space exploration and to less diverse solutions when compared to both human-only and computer-only search. This has important implications for future human-computer collaborative design systems.