In this study, extremely highly plastic soil samples were collected from clay deposits in the Akyurt district of Ankara (Turkey), near Esenboga Airport. Hydrated lime and fly ash were applied as chemical stabilizers. The clay samples were mixed with a stabilizing agent considering the dry weight (lime content: 1%, 3%, 5%, 7%, and 9%; fly ash content: 5%, 10%, 15%, 20%, and 25%). The engineering properties of the improved soil samples were determined via laboratory tests. The changes in engineering properties, especially in regard to the swell potential and compressive strength, were analyzed in this paper. Correlations between index properties, swell percentage and swell pressure were established. The effective lime content reducing the liquid limit and plasticity index ranged from 4 to 5%, whereas approximately 8% lime was necessary to attain an allowable swell percentage. Fly ash did not effectively improve the extremely highly plastic clay in reasonable amounts (i.e., 20 to 25%) in terms of its swelling properties.