One plan sees the capacity increased to 54,000, and the other to 60,000 which would make the stadium the third largest among England's league clubs.
Manchester United's Old Trafford is the biggest with a capacity of almost 76,600 followed by Arsenal's 60,400-seater Emirates Stadium.
Newcastle United and Sunderland both have bigger capacities than City, while work is expected to start on Tottenham's new 58,000-capacity White Hart Lane stadium later this year.
"Games are selling out frequently prior to going on open sale and there is rising demand from supporters for more season tickets, match-day tickets and premium seating," City club spokesman Pete Bradshaw said in a statement on the club's website (www.mcfc.co.uk).
"This is why we are considering these expansion proposals."
City's former ground Maine Road still holds the attendance record for the largest crowd ever to watch a match in Britain outside London or Glasgow with 84,569 at an FA Cup tie against Stoke City in 1934.
It was demolished after City moved to the Etihad in 2003.