The lack of a computerized approach to optimally size hybrid Ground Source Heat Pump (GSHP) systems leaves a glaring gap in our knowledge of their benefits when utilized in an urban environment. To address this issue, this study introduces two new, robust methodologies, one for determining optimal GSHP size as part of a hybrid system for an individual building and one for assessing the appropriateness of combining multiple buildings onto a single hybrid GSHP system. Both methodologies simultaneously consider heating and cooling and are applicable to residential, commercial and industrial buildings that are either heating or cooling dominant. Using these methodologies can result in significant reductions in initial costs of installation, payback period, and operation costs, when compared to following rules of thumb or using non-hybrid systems. In most cases, when optimization is performed, the GSHP meets very large portions of the total annual heating and cooling demand of buildings.