Although Doncaster offered Cotterill a new contract at the end of the 2013–14 season, he opted to move somewhere closer to his family in Wales. He signed a two-year contract with Championship club Birmingham City, and made his debut in the first-round League Cup win against Cambridge United on 12 August. He became a regular selection, and scored his first goal for the club on 27 September in a 2–1 defeat at home to Fulham. After Callum Reilly won the ball in midfield and slid it out to the wing, Cotterill cut inside a defender and curled a shot into the far top corner. In the next match, away to Millwall, he set up goals for Clayton Donaldson and Wes Thomas as well as scoring himself with a swerving free kick from 25 yards. Cotterill established himself as a regular member of the starting eleven, and finished his first season with 9 goals from 45 appearances in all competitions. Manager Gary Rowett highlighted his rare ability to "play badly and still affect the game in a positive way", such that "even when he is not on song he can still affect the game with a little bit of quality", as indicative of his importance to the team. In the 2015 close season, he signed a two-year extension to his contract.