Extending the waistline in a pair of trousers with a fly front can be accomplished in a number of ways. Some are very simple and do not require taking the pants apart; others are more complicated because they will require opening seams, resewing seams, occasionally adding fabric to extend the size. Altering the length of the waistband in jeans is not advised.
Fit the trousers to the wearer
Determine how much extension is needed to make pants fit properly and comfortably by trying them on. This could also be accomplished by measuring the waist of the wearer as well as the waist of the trousers.
Plan a short extension
If the extension needed is an inch or less, the addition can be made by moving the fasteners along the waistband. This method works well for trousers that close with buttons and for those with metal hooks. Experiment with this method to be sure the closure continues to align with the zipper. If the alignment is too far off, choose another method.
Observe the back seam
Inspect the back seam of the trousers to see how much seam allowance has been cut in the construction of the trousers. If it tapers up from the crotch to two or more inches at the waist, the alteration of the pants will be straightforward. If there is little or no seam allowance, fabric that matches the pants will need to be borrowed from another area, like the inside of a pocket or the allowance turned up in a hem.
Observe the construction of the waistband
If the back of the waistband is joined with a seam, the alteration is simplified, but unless there is a generous seam allowance there, matching fabric will have to be borrowed from another area. If there is no seam, even more fabric will be needed.
Snip the seams
Use a seam ripper or sharp scissors to snip the stitching that holds the seams. Open the seams that hold the waistband onto the pants for at least four inches or more from the center on both sides. Open the back seam at least four inches down from the waist.
Reconstruct the back of the pants
If the seam allowance is adequate to make the pants larger at the waist, resew the back seam securely and press. If there's not enough width to extend the size of the pants, insert a matching piece of fabric cut into a V shape into the seam. It will need to be as wide at the top as the extension needed. Sew each side securely and press.
Reconstruct the waistband
For a waistband with a seam, use the seam allowances, if possible, to extend the band and stitch it vertically. If seam allowances are not adequate, insert a rectangle of matching fabric into the waistband. It will need to be as wide as the extension needed. Be sure to add to any facing or interfacing to the insertion. Sew securely and press.
Sew the waistband onto the pants
Sew the seam between the pants and the waistband as it was sewn originally. The raw edge of the top of the pants should be sandwiched between the layers of the band. Most waistbands are topstitched.
Finish the alteration
Steam and press the back seam again. Press the waistband thoroughly.