According to Square Enix if you have the Final Fantasy XIV base game (A Realm Reborn) or the FFXIV Online Starter Edition you will be accessed to the Heavensward expansion. Add it to your account for free and keep it forever! The offer is valid for PlayStation 4, PC and Mac platforms. Those who have Heavensward will get access to all updates, launched through 3.5: The Far Edge of Fate.
More to Note
“Heavensward being free coincides with the ongoing Free Login Campaign, which runs until 17th June and gives lapsed players seven free days of gametime to come back and enjoy Eorzea. The offer gives players the perfect chance to jump back into the world of FINAL FANTASY XIV Online ahead of the launch of its upcoming third expansion, Shadowbringers.”