When adding seam allowance to pattern pieces there are two issues to consider:
- How much seam allowance to add, and
- How to finish off the seam allowance corners; i.e. do you want the cut edges to line up?
You do not have to add the same amount of seam allowance on all seams; in the garment manufacturing industry is is common to add less seam allowance to necklines and armholes, whereas quite often commercial patterns have the same amount of seam allowance for all seams.
How much Seam Allowance?
Seam allowance added in fashion production is usually less than added to Commercial Sewing Patterns. It also varies from manufacturer to manufacturer, but a rough guide would be:
In Fashion Production:
- 1/2-inch on standard seams
- 1/4-inch on curved seams such as necklines and armholes
- 3/4-inch for zips
In Commercial Patterns
- 5/8-inch on all seams (not including hem)
Of course when making your own patterns, you can choose what you add. If you are new to patternmaking, it makes sense to make what you are used to; if you do a lot of sewing with commercial patterns, then it makes sense to stick with that. If you are learning patternmaking for fashion production, use industry standards.
As per the information above, you do not have to add the same amount of seam allowance to each seam; e.g. you can add less to the neck and armhole than the side seams and waist.
How to finish off Seam Allowance Edges?
When sewing two pattern pieces together, you sew on the seam line, not the cut edge. Depending on how you do your seam allowance, the cut edges may or may not line up. While it doesn't really matter whether or not the edges line up, it's a bit easier to sew if the seam allowance corners so DO line up. However, making the cut edges line up take a bit more work. What you need to do is square the corners off rather than leave them at strange angles.
If you do want your cut edges to match up, you need to keep in mind that one seam may be sewn onto two pattern pieces, and therefore it cannot be squared off to both; you would have to keep in mind the order of garment construction and square off the first seam to be sewn.