In Phantom Menace, we see Obi-Wan Kenobi defeating (Darth) Maul by slicing him in two halves, after that Maul is presumed dead.
However, in The Clone Wars series, he appears to have survived this blow, by clenching to the Dark Side of The Force.
Was his survival planned, or is it a retcon added in The Clone Wars? I did not find any hints to Maul's survival in the Phantom Menace official novelization by Terry Brooks.
An official statement by any Lucasfilm or Disney staff, is a perfect answer.