Fellowes' plans, reported on the BBC News website (the show airs on ITV in the U.K.), will be to look at the pair's early courtship from the American heiress' arrival in the U.K.
The spinoff drama will cast a pair of younger actors in the roles played by Bonneville and McGovern. Some of the drama may be set in America.
Fellowes said the spinoff would be broadcast after the end of DOWNTON ABBEY, which airs on PBS in the U.S.
"I don't think you can continue a narrative in more than one area at once," he explained.