The ground-source heat pump (GSHP) system using a horizontal ground heat exchanger (GHE) can be employed to reduce the installation cost and obtain a balance between efficiency and costs. Among the different types of horizontal GHEs, a spiral-coil-type GHE is one of the advantageous configurations in terms of thermal performance. However, there is no satisfactory guideline to design the horizontal spiral-coil GHEs, though design methods for other types of horizontal GHEs exist. Hence, in this study, a design method is proposed for the horizontal spiral-coil GHEs by modifying the boundary conditions of an existing equation. To verify the applicability of the proposed design equation, a laboratory thermal response test was conducted to validate the finite element model. Then, the validated numerical model was utilized for a computational fluid dynamics (CFD) simulation on an arbitrary building wherein a GSHP system with a horizontal spiral-coil GHE is operated. The entering water temperature (EWT) of 32.09 degrees C from the simulation result was lower than the design EWT criteria of 32.2 degrees C, implying that the thermal performance of the GHE for a month of operation is sufficient to cover the building load. The result provides the applicability of the proposed design method.