There has been an increased interest in reducing the cost and environmental impact of building heating, ventilation, and cooling systems by using hybrid renewable energy systems. Among them, heat pumps which combine geothermal and solar thermal energy have gained attention due to their high efficiency and reliability. However, such systems can have high install costs. It is therefore important to design them in an economically optimal way, and to evaluate and compare them to conventional solutions over the full system life cycle. This paper presents a detailed study into the optimal system operation and design of a Solar-Assisted Ground Source Heat Pump system. The design variables include solar collector area, borehole depth and volume of the thermal storage tank. The operation and design methodology is demonstrated using data gathered from a real system in Melbourne, Australia. For this system and location, the outcome is that an optimally designed Ground Source Heat Pump system should cover approximately 90% of the total heating demand, with the remainder covered by conventional sources. The approach can be applied in the same way to other systems and other geographies.