McEwan says he is working on an adaptation with composer Michael Berkeley and poet Craig Raine.
He told Friday's Times newspaper he wanted it to be on a grand scale, saying "it's not a chamber piece, that's for sure."
"Atonement" travels from an English country house to World War II France and centers on a love affair doomed by a child's misunderstanding.
The 2001 novel was shortlisted for the Booker Prize. A 2007 film adaptation starring Keira Knightley was nominated for seven Oscars and won one, for music.
The newspaper says opera houses in England, Germany and the U.S. are in talks about co-producing the piece for a possible 2013 premiere.