The California Public Utilities Commission approved the new area code Thursday as part of an "overlay" plan for the same geographic region as the existing "415" area code.
New telephone numbers will be issued the "628" area code when new phone services begin as of January 2015.
The "415" area code in Marin and San Francisco counties is expected to use up all available "prefixes," which are the first three numbers after the area code, by October 2015.
In anticipation of that demand, the North American Numbering Plan Administrator approved an area code overlay to provide additional numbers.
Residents in the "415" areas will not have to change their phone numbers under the overlay system -- multiple area codes can co-exist within the same region.
In preparation for the new area code, a new dialing procedure in the "415" region will require dialing the area code to be connected.
Dialing "415" within the same area will be required starting mid-June, however, there will be a grace period until mid-December 2014.
Earlier this year, the CPUC held four community meetings to garner input about introducing a new area code to the "415" region.