Length is key to perfect fit, and luckily, it's easy to add or subtract length to top and dress patterns via a few different methods. Start by noting where other design features, such as the waist seam, land on your body — that'll determine if you need to lengthen or shorten within the pattern pieces, or if you can just alter the hemline.
Remember that your final adjustments might not be based on your height, which may seem counter-intuitive! Consider the whole design, and your vision of the final garment.
Method 1: Lengthen or shorten at the hem
For tops without shaping at the waist or horizontal seams, it's fast and super easy to add or take away length at the hemline. This is great for shaped hem edges, like the shirttail hemline on a button-front shirt in the image below.
Be sure to make the same change on all necessary pattern pieces. It's easy to focus on the front and back pattern pieces, but don't forget facings and button bands. I like to lay out all the pattern pieces and double-check that I've made the length change on everything before cutting.
It's possible to add the extra length right on the fabric when you're pinning and cutting, of course, but that's when you can easily forget to add length on to an auxiliary piece like the facing — so, though it takes an extra minute, I do recommend making the change on the pattern pieces first. Then you can place the pattern pieces on the fabric and cut out with total confidence.
Method 2: Use the lengthen/shorten line on the pattern
This method is best for patterns that have more shaping in the bodice; if you were to adjust the length of your pattern without adjusting the waist, it might fit awkwardly. By using the marked line, you're more likely to get a better fit.
Most patterns have a note about where to lengthen or shorten, which makes this adjustment easy. In standard pattern notation, the "lengthen or short line" (L/S line) is a double line — choose one of these lines to use as a reference and ignore the other.
Shortening using the L/S line
Determine how much you want to shorten the piece. In this example, I'm shortening the bodice by 1".
Draw a line 1" above the lengthen or shorten line, making sure it's perfectly parallel. You can see my line in the image above, in red.
Crease the pattern on the lengthen or shorten line. Then bring the crease up to the drawn line, which removes that amount of length to the pattern piece. Flatten this new fold and pin or tape your pattern so the fold stays put.
Make the same adjustment on the other front and back pieces. On a piece with princess seams, you'll have a number of pieces to adjust. For a garment with darts, use the dots at the widest part of the dart and draw connecting lines to the end. The dart will be slightly different, but you won't be able to tell on the finished garment.
Lengthening using the L/S line
Do the same thing, just in reverse. To make a piece longer at the shorten or lengthen line, you'll cut the pattern and add an insert of tissue paper or other lightweight paper.
First, draw a few perpendicular lines across lengthen or short line, indicated in the image above by the blue arrows. These lines will help you keep the pattern aligned when you add the insert.
Determine how much length you want to add. For this example, I'll add 1¼".
Cut the pattern piece apart at the lengthen and shorten line. Tape or pin a piece of paper that's as long as the length you want to add (my paper is 1¼") to the upper piece.
Draw a continuation of the perpendicular lines you made in Step 1 across the insert paper.
Use the perpendicular guidelines to align the lengthened upper piece with the lower piece, tape or pin in place. You'll need to draw in the side seam using a curved ruler (or freehand; adding this amount won't change the edge too much).
Finally, redraw the dart or any other markings on your pattern. In this example, I would move up the bottom dot and return the whole dart to its original length. Be sure to make the exact same length change on all the necessary pattern pieces. Now cut out that gorgeous fabric and get sewing!