The Witcher 3's first expansion, Hearts of Stone, was announced months ago, before the release of the game, alongside another expansion called Blood and Wine that's not due until next year. Hearts of Stone is out next month, but until now we didn't know much about how it'll fit into the game, only that it'll add about ten hours and take place in No Man's Land and Oxenfurt.
As spotted by VG247, however, CD Projekt Red community lead Marcin Momot has been answering player's tweeted questions about the game. He told one that while Hearts of Stone can be played during or after the main story of The Witcher 3, the quests are aimed at those at level 30 or above.
@savroRus You can play it after/during the main story. The HoS quests will be level 30+ though.September 7, 2015
To anyone who's played a particularly large amount of The Witcher 3, don't worry: Momot also says the quests in Hearts of Stone will scale up for New Game Plus.
@Zonkil9 Quests in HoS will scale up for NG+, yes.September 7, 2015