You may make changes to the armhole that will effect the fit of the sleeve cap

For example, if you extend the armhole, you will also need to extend the sleeve cap. 

If the change is by a large amount you should use the cut and spread technique, to distribute the extra length evenly throughout the cap (between the notch and the side seam, on both front and back of the sleeve), but if it is only by a small amount (a few centimetres), this method will do the trick.

extend bicep line_1

Take the sleeve you are altering and trace it onto a new piece of pattern paper - leaving space around the outside for changes and seam allowance.

extend bicep line_2

By this stage, you should know what the difference between your sleeve cap and armhole is.

Take the difference and divide by two (you will be distributing it evenly on both ends of the bicep line) and extend the bicep line by this amount at either end, ensuring that the bicep line remains perpendicular to the grain.

extend bicep line_3

Redraw the sleeve cap, passing through the balance notches before gradually steering the line towards the new underarm points).

extend bicep line_4

Redraw the sleeve side seams by joining the underarm points to the wrist at either side.

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