Elfreth's Alley is known for it being the nations oldest residential street, dating back to the very early 1700s.  There arent an permits needed.  They just ask for a curtesy email.  It can be a busy spot with tourists especially on weekends, so it works better for engagement sessions.  There is a little smaller alley that works nicely for a shot or two as well just off of Elfreth's.  It sometimes works nicely for couples that are using Race Street Pier for the sake of proximity and getting two different looks of Philly.