If I have some existing code which is not buggy (hopefully) and I'm just looking for some design insights about its extension then do I ask on SO or on CodeReview?
I have some rudimentary ideas about the extension but no code yet.
Definitely not SO. - if insights it's what you are looking for - it's off topic. You need actual code for code review - so it's off topic there too.
I would go with programmers - it will provide you a platform to ask more conceptual questions like design patterns etc.