Site effects are resulted from the non-linear filtering mechanisms within the soil sedimentary layers overlying bedrock. In contemporary design codes, site effects are taken into account by introducing different site factors for different site classes. The prescribed site classification systems are based on averaging shear wave velocity in the soil sediments. However, significant amplification of the seismic displacement demand may be developed from mechanisms which can result in resonance behaviour. In such situations, soil amplification cannot be determined accurately by considering the average shear wave velocity of the sediments alone. The effects of vertical heterogeneity in the soil sediments have not been explicitly parameterised in the conventional code provisions. This paper presents results from parametric studies showing the influence of vertical heterogeneity in the soil sediments on the soil amplification behaviour. A methodology for modelling soil heterogeneity is described. Importantly, the presented results quantify the influence of vertical heterogeneity on the seismic soil response behaviour. It is found that variations in the sub-soil layer properties can accentuate soil amplification by up to 1.6 times. Comparisons with previous research results revealed consistencies in the findings. It is expected that information presented in this paper would be useful for engineering design applications.