How do you add width to a pants pattern that has no side seams?

Some athletic (sweat) pants and pajama pants are basically two pieces -- the right and the left, that are sewn together.

If that is the type of pattern you have, fold the paper pattern in half and make a crease from waistline to the hem on the paper pattern. Unfold the pattern and cut down the crease or fold line, from the waistband to the hem of pants on the pattern.

Now pin the CUT pattern on the fabric you wish to sew, but space the two pieces out so there is room between them. If you want the waistline 4 inches wider, for example, you will want the two paper pieces to be at least 2 inches apart (plus extra for any seams.) The additional fabric you gain is muliplied by TWO as you are cutting two pants pieces: a right and a left. You can add a piece of additional paper and tape into place, but there is no reason you have to. Simply place the paper pieces on a two-ply of fabric, use a ruler to be sure the two pieces are spaced equally apart the whole way down, pin the pattern pieces into place, and then cut the fabric. The notches, grain of fabric, etc. all stay the same. The only thing you are changing is the width of the pants.

You will need to add a length the the elastic waistband as well. If the waistband is a separate piece, it needs to be cut longer -- the same additional length as you added to the pants pattern.