Expansive soils, notably black cotton soil (BCS), present substantial challenges in geotechnical engineering due to their propensity for volumetric changes with varying moisture levels. The present study conducts a comprehensive comparative analysis of the consolidation properties of BCS when stabilized using rice husk ash (RHA) and coconut husk ash (CHA). Aimed at identifying the most effective bio-waste soil stabilizer, the research scrutinizes several pivotal parameters, including free swell index, linear shrinkage, heave, void ratio, swelling pressure, and compression and swelling indices. Experimental data demonstrates that both RHA and CHA significantly enhance the geotechnical properties of BCS. Both the ashes exhibited a favorable influence on the swelling characteristics of the soil, but CHA is markedly superior in mitigating shrinkage and improving the swelling potential of BCS. The compression index was reduced by nearly 44% and 57% when the soil was treated with RHA and CHA, respectively. The findings of this study underscore the potential of both RHA and CHA as sustainable soil stabilization agents, with each demonstrating unique advantages. Furthermore, the research underscores a nuanced understanding that a material like RHA, renowned for its potent pozzolanic attributes, might not inherently excel as a good filler material.