Ground-source heat pump (GSHP) systems coupled with borehole heat exchangers (BHEs) are widely used as a renewable energy source. However, the high initial costs to install the BHEs still acts as an obstacle in the expansion of these renewable energy source systems. Specifically, in South Korea, a typical residential type corresponds to an apartment building with a high building-to-land ratio for land efficiency, and thus the space to install the BHEs is usually insufficient. Furthermore, the high initial cost issue of BHEs makes it difficult to ensure the feasibility of GSHP projects in this type of a situation. This study proposes a novel BHE sizing method to support the process of sizing energy sources in the design development phase of a construction project. Life cycle cost (LCC) analysis was combined with a tool to optimize BHE sizing by considering various economic aspects. Entering water temperatures (EWT) to heat pumps in conjunction with the LCC were used to define objective functions. Consequently, Pareto optimal solutions were obtained on the EWT–LCC plot. A group of Pareto optimal solutions forms a Pareto-curve, and each point on the curve indicates a possible sizing scenario or alternative. Finally, it is possible for decision makers to compare the solutions that include both technical and economic information. The Pareto optimal solutions are expected to support proper decision making in the early design phase.