Angle and amplitude estimation errors in magnetic direction finding, called site errors, are important sensor-specific errors in lightning location systems (LLS). They are known to be caused by nearby cables and overhead lines due to induced currents. Due to the reflection and diffraction of electromagnetic waves, hills and mountains are also expected to generate these effects. In this paper, numerical computations applying the finite-difference time-domain (FDTD) method are performed to analyze the impact of hills or mountains on the angle and amplitude estimation of LLS sensors for typical first and subsequent return strokes (RS). The influence of hill size, the distance of sensors to the hill, ground parameters, and sensor bandwidth, are evaluated. The results show that on top of low ridges of only 125 m elevation, up to ±10° angle site error and +50% amplitude site error occur. A key finding is that due to field attenuation caused by lossy ground and sensor bandwidth limitations, there is practically no difference in the angle site errors for first and subsequent RS. For two sensors, the average site errors obtained from real sensor measurements are compared to results from 3D-FDTD simulations, modeling the real terrain based on a digital elevation model. The simulation results are in good agreement with the observed, average angle site errors.