The common types of hemline alterations include traditional hemming (shortening or lengthening the dress evenly), layered hemming (adjusting the hemline of gowns with multiple layers), asymmetrical hemline (creating a unique and modern look by altering the hemline to be higher on one side or featuring a high-low design), and scalloped or lace trim (adding or removing delicate lace trim or scalloped edges to enhance the hemline).