This paper introduces a new sizing methodology for ground coupled heat pump (GCHP) systems which takes into account groundwater flow (by using G-functions based on analytical models) in order to achieve an economic optimization of the total cost of the project. The procedure includes the calculation of the initial and the annual operational costs. Optimal design variables (boreholes depth, distance between consecutive boreholes, etc.) and borefield layouts (number of boreholes in the x -direction) are presented for different values of the thermal conductivity of the ground. In addition, a parametric study is done to measure the impact of the groundwater flow velocity and angle with respect to the borefield on the economics of the project. It is shown that the effects of the groundwater flow velocity on total cost become apparent only for high velocities, i.e.,