This is a simple way to get 3/4 to 1 inch more out of a too-tight waistband.
Make a fabric tab slightly smaller in width than the waistband by about 1.5 inches. Unstitch the waistband center front seam, slip the tab inside leaving about an inch or more showing. The tab size will depend on the width of waistband and size of snap or button. Top stitch the seam closed. Apply the upper snap or buttonhole (you can do this before inserting tab into waistband) to the tab.