The administrative parts. And those are the parts the folks who ask for new features always forget.
Them: "Hey, we want a new feature to do X".
Me: "Ok. That will require a new table or two in the database, the front end screens, and we have to double the time because I have to build the back-end administrative screens for you to manage the setup for the feature."
Them: "Double the time?!?!?!"
Me: "You DO want to control the setup, right?"
Them: "Of course."
Me: "Then we have to build the administrative screens, too, which will require their own design and user interface and validation, etc."
Them: "So, when can you get it done? Next week?"
Me: *sigh*