This paper reports a design and building process of a net-zero-energy modular house, named ElementHouse, which was entered for the 2007 Solar Decathlon competition that was organized by the U.S. Department of Energy. This paper discusses the development of the ElementHouse from its initial concept to the actual construction, which integrated multi-disciplinary knowledge of architecture, mechanical engineering, and electrical engineering. By employing computer-aided simulation tools, several design approaches were developed to achieve the optimal balance among function, aesthetics, economy, and energy—a challenge in many sustainable building designs. A simplified energy model helped to form the building configuration at the preliminary design stage by showing how energy use is affected by various parameters, leading to optimization studies that provided design guidelines towards an energy-efficient building envelope and opening design. Energy modelling also estimated the annual energy use and electricity generation, as well as the costs associated with long-term operation of the house. The energy demand estimate and its daily power profile helped to design the photovoltaic (PV) system. The modular configuration of the building and its roof structure was then iteratively altered to accommodate the PV panels in such a way as to generate the most power and to facilitate interstate transportation of ElementHouse. With the progress of the design process, detailed energy simulation using EnergyPlus provided more accurate estimates of power use and generation and, coupled with daylighting simulation performed with Radiance, permitted finalizing the building envelope, opening, and electric lighting design. The building envelope embraced the passive and low energy philosophy to minimize the annual heating and cooling load and to optimize natural lighting. By illustrating the whole picture of ElementHouse design, this paper discusses a way of effectively designing and building a comfortable and affordable solar house.