Things You'll Need
Flexible tape measure
Dark felt-tip marker
French curve ruler
Clear graph ruler
Fabric (approximately ½ yard)
Serger or overlock machine
Working with a professional tailor to add width to a formal dress will ensure a perfect finish as well as avoid damage to delicate fabrics, but making do-it-yourself adjustments to unlined knit dresses will save money. Measurements such as the bust, chest, and waist will act as guidelines for the expanded pattern. By adding a side-seam insert panel and adjusting the underarm, the size 16 dress is transformed into a size 20. Certain dresses do not include excess fabric at the hemline and a similar or complementary fabric will have to be used for the insert panel.
Measure your shoulder width, chest and waist with a flexible tape measure. You will be using these specifications as your guide for your side-seam insert panel pattern. Although one inch generally is added all around for each increased dress size, not all body shapes increase in the same proportion.
Turn your garment inside out and look at the added seam allowance at the side-seam and bottom hem. Side-seams generally have an added 1/2 inch allowance which is not enough fabric to increase the dress. Certain dresses will have a three-inch hem border for hem adjustments. Open the hem with a seam ripper and cut 2 1/2 inches around the hem with your fabric scissors. You will use this extra fabric to construct the side-seam insert panel.
If your hem is a ½ inch hem, you will need to locate similar or complementary fabric to increase the dress at each sideseam.
Lay your dress flat on your pattern paper. Carefully trace around the area you want to increase with tailor's chalk avoiding any chalk lines on the dress. For instance, if you are increasing the bodice and leaving the full skirt intact, make sure to outline the shoulder, side-seams, armhole and waistline seam.
Remove the dress and define the lines with a dark felt-tip marker using a ruler or French-curve ruler.
Place a clear graph ruler over the front side-seam area and increase the pattern by one inch on each side. Mark the inch with your drafting pencil on your pattern paper and shape with your rulers. Add an extra 1/2-inch seam allowance. Repeat with the back pattern.
You will have a two-inch increase at the front and a two-inch increase at the back for a total four-inch increase altering the size-16 dress to a size 20. Keep in mind that you will have to make adjustments to these specifications according to your body type and dimensions taken in Step 1.
Shape the top of the side-seam pattern according to your sleeve's underarm with your French-curve ruler. Keep in mind that the added side-seam will be inserted and stitched into the underarm armhole. You will need to seam-rip open the underarm panel to stitch the side-seam insert. This will naturally give more room at the armhole.
Cut out your pattern with your scissors.
Pin the patterns to your dress fabric with straight pins. Make sure to pin in the seam allowance area for delicate fabrics to avoid pin-holes. Cut out with your fabric scissors.
Machine stitch or serge the front and back side-seams together. The type of machine used will depend on the inner construction used in your garment. "Choose the 3-thread overlock for loosely fitted or nonstressed seams…" according to "The Complete Photo Guide to Sewing." Tailoring includes duplicating the dress' inner construction for a clean and professional finish. You will have two side-seam inserts, one for each side.
Pin the side-seam inserts to the dress, with the wrong sides facing out. Turn your dress inside out and carefully try on. As an alternative, baste the side-seam panel inserts with your hand-sewing needle and thread. Make any necessary adjustments. Often times the added four inches will be too big at the natural waistline curve. Re-pin or baste until you are satisfied with the shape.
If the skirt's waist seam has fullness gathers and bunches up at the side-seam, you will have to carefully seam-rip open the gathers to spread the fabric evenly with the side-seam inserts on each side.
Machine stitch or serge the inserts. Make sure you switch your stitch selector to the same type of stitch used for the dress.