The first of two Dark Souls 3 add-ons is due for release this fall.
First Dark Souls 3 DLC to arrive this fall
According to Videogamer, the first add-on is due to launch this fall. We don't have any information as to the content of this DLC, and there likely won't be any for some time.
The second add-on remains without a release window. Dark Souls 3 does have a season pass, priced $25 that includes access to both add-ons.