There is some conflicting information on the web about the Sejongno Immigration branch. Some sites are still posting the old address near Anguk Station and I even found an address for the new one that is a few blocks off that can get people turned around. On the government website it is still an additional click about the change, even though it is over a year old. If you are new to  renewing your visa, recently moved or are moving to North Seoul from another branch, or have forgotten then this is for you.