Amelia's Atelier (or simply the Atelier) is a room inside the Croft Manor. Lara discovers it in the Rise of the Tomb Raider: 20 Year Celebration DLC Blood Ties, when she finally enters the West Wing that her father declared off limits after the death of Amelia on her voyage to Tibet.
The Atelier can be found on the first floor through a closed door that leads to the West Wing. Lara needs to find the Master Key first and open the door. Here she will find a tree that crashed into the building some time ago. Past that tree down the corridor to the right the Atelier is located, while directly ahead there is the Master Bedroom.
The Atelier is filled with paintings done by Amelia - among loads of brushes, paint, and books. It seems that she was finishing some of her art before she went on her trip to visit Richard in Tibet.
Lara has to search the Atelier for clues to the combination of the Safe in Lord Croft's Study. As he could not remember the combination, he left himself clues, one of which is "My favourite painting by Amelia...". From a document located in the Atelier Lara can conclude that it is the abstract with the red squares. She simply has to count the red squares displayed on that painting, a Relic which she can find close by.