This study evaluates the effect of costus lateriflorus bagasse ash on expansive soil properties. Laterite and clay samples from the Ubeta-Ula-Ubie road at Ahoada West LGA in Rivers, Nigeria were prepared and laboratory analysis performed for development potential, volume change, maximum dry density (MDD), optimum moisture content (OMC), consistency limits, California bearing ratio (CBR) and unconfined compressive strength (UCS). Initial analysis classified the soil as A-7-6 under the AASHTO classification system. The development potential, volume change, maximum dry density (MDD), liquid limit (LL), and plasticity index (PI) of laterite and stable clay on the Ubeta-Ula-Ubie road decreased with increasing proportion of bagasse Costus ash lateriflorus, while optimum moisture content (OMC), plastic limit (PL) and unconfined compressive strength (UCS) were increased by the addition of Costus lateriflorus bagasse ash. The results showed that increasing the ash content of bagasse increased the soil properties suitable for road construction. However, bagasse ash performs better on lateritic soils than on clay soils, and 7.5% bagasse ash would be suitable for use as a stabilizer in soils with similar properties to laterite and clay soils in the Ubeta-Ula-Ubie road.