Making a house into a duplex legally?

[Jeff Something]

Sometimes when browsing houses for sale, I'll see the term "legal duplex," which would obviously seem to imply that there is such a thing as an illegal duplex. So, my question is, if I had found a single family house that I thought would make a wonderful duplex (obviously taking into account what I would imagine is a large construction project), where would I go to find out the rules and regulations? Is there generally a specific place that I can go and ask, or better yet, get a copy of stuff like this?