Didn't hear back from SO for two days so I had to Google it (imagine that).
This article covers code organization tips including this one about admin sections:
Almost everyone puts the administrative files under admin, when it’s needed. If you have an admin section, do the same. Don’t have duplicate code, images, JavaScript, etc. for the admin. Obviously, for the parts of the admin section that are different, you’ll need to have additional code etc. But it should be part of the same codebase, and factored such that you can use the helper functions from any part of the website.