This research study is focused on stabilizing black cotton subgrade soil (BCS) predominantly found in many locations across western parts of Maharashtra, India. The study uses locally accessible materials such as cow dung and sugarcane bagasse. It has been used in the form of cow dung ash (CDA) and sugarcane bagasse ash (SBA) to substitute black cotton soil to improve its strength partially. The CDA and SBA were used for partial soil substitution in the ranges of 2.50%, 5.00%, 7.50%, 10.00%, 12.50% and 15.00%. The obtained results were checked with the original naturally available BCS to find the optimum content. The optimum moisture content rises, and dry density falls with the stabilization process. The trend of California Bearing Ratio test (CBR) increases and peaks at 10.00% partial substitution of soil by CDA and 7.50% partial substitution of soil by SBA, respectively and decreases for the following values. Hence, the optimum content is calculated as 10.00% for CDA and 7.50% for SBA, which shows substantial improvement in the CBR value, and helps to control the changes in volume. The CBR value for the optimum combination is augmented by almost 4 to 4.25 times the original CBR value of the BCS. The difference in pavement thickness is analyzed for the three conditions, and site-specific design thickness charts are produced and recommended for the improved soil subgrade using IITPave software.