The 20-year-old, who has represented Northern Ireland at senior level, has racked up 31 appearances since making his debut in November 2010 and the midfielder could be set for a bigger role at right-back under boss Lee Johnson.
"I am really pleased that Carl has signed his contract extension and is staying," Johnson told the club's official website.
"I felt bad for Carl last season because he didn't have enough game time when you consider how well he trained and played in reserve-team games.
"I know Carl wants to play in midfield, but I want to have a look at him at right-back because I think he has the potential to be a top class right-back."
Meanwhile, Johnson is running the rule over a number of trialists including Senegalese striker Momar N'Diaye and an unnamed Latvian centre-back.