Seam allowance – do you include it on your pattern or not? Generally in fashion garments the seam allowance is drawn into the pattern. However, I have a costume background and in this industry it is added on only when you cut out the fabric. In terms of size and when you add it you really can use whatever seam allowance you prefer. The purpose of a seam is simply to join two pieces of fabric without exposing the raw edges. The seam allowance is just the extra fabric so the stitching isn’t too close to the raw edges and doesn’t come undone and cause fraying.
I prefer adding on the allowance as I cut as it makes it easier for me to alter the patterns to different sizes at a later date. If you add it to the pattern however, accuracy in the seam allowance size is easier to achieve. Whichever you prefer it is always best to mark your stitch lines on the garment to achieve the best possible precision as you sew. If you are worried about fit a bigger seam allowance will give you some leeway to alter – and you can always trim it down at a later stage!