Things You'll Need
Knitting needles, those used for the body of the sweater
Knitting needles, smaller size than those used for the body of the sweater
Lengthening a hand-knit sweater is possible even if the sweater was knit from the bottom up. Top-down knits are easily lengthened by simply removing the bind-off edge, or the last row, attaching yarn and continuing to knit in the previous pattern until you reach your desired length. Bottom-up knits and knits with hem stitches are a little more difficult. These require removing several rows. In the case of sweaters knit from the bottom up, the cast-on row must be clipped to allow you to knit the extension in the same direction as the body of the sweater. Otherwise the extension stitches appear out of line with the rest of the sweater.
Lengthening Top-down Knits
Remove any seams if the front and back of the sweater were sewn together rather than knit as one continuous piece.
Clip the knot that binds the very last cast-off stitch. Gently pull the freed-yarn end and unravel the first bind-off stitch.
Insert the tip of a pair of circular knitting needles the size used to knit the sweater into the loop that's revealed when the bind-off stitch is pulled out. Slip the loop onto the needle with its right leg forward. Pull the rest of the bind-off stitches out one at a time, inserting the knitting needle into each loop that's revealed when the bind-off stitch is pulled out. If the sweater's hem contains any hem stitching, such as a band of garter stitch, unravel the entire hem band. Begin slipping stitches back onto the knitting needles once you have unraveled back to the stitch used for the body of the sweater.
Examine the stitches on the circular needles and ensure each one is right leg forward so your extension is knit in the same direction as the rest of the sweater. If any stitches are sitting on the needle in the wrong direction, slip the stitches in front of the offending stitch onto a separate needle or pair of circular needles, slip the offending stitch off, turn it and slip it back on. Slide the rest of the stitches back on to the working needles.
Attach new yarn at an appropriate place, such as a side seam or at the sweater's back. Knit the extension following the pattern used just prior to binding off. Knit to the extension's desired length. Bind off as directed in the original pattern.
Lengthening Bottom-up Knits
Remove any seams if your sweater is sewn together rather than knit in the round as one piece.
Decide where you want the extension to begin and mark that row with a long straight pin. Any point in the sweater's body is appropriate as long as it is above the cast-on row, as you cannot add an extension directly to a cast-on stitch. If the hem includes any border or hem stitches, such as a band of garter stitch, choose a place above this band to begin so the entire band of hem stitching is removed.
Pick a place to start and clip a stitch two rows above your pin. Rotate the sweater and hold it hem up.
Pull the cut yarn to slowly unravel it from the stitch above it. Insert the tip of a small knitting needle into the loop that's exposed as you pull the yarn free. Continue until the cast-on rows are removed and the small knitting needle holds all of the sweater's stitches. Slip the loops on to the needle right legs forward to keep the direction of the stitches uniform.
Join new yarn to the sweater at a side seam, or in the back, and knit the extension to the desired length. End and remove the needles with the standard binding-off stitch.