Has anyone added extra booty room?

I have just made up a pair of size 10 picnic shorts and after trying them on her, they are a little snug around the bottom. Otherwise the fit is fine. I made them from a stretch woven but it still too tight. I am not sure how to alter them, should I just slash and spread (which is my usual method) and if I do, where? and how do I alter the corresponding facing panel?