This side quest becomes available after you gain access to the Room of Requirement which happens during the Complete the First Keeper Trial main story objective. Professor Weasley will task you with gathering some moonstone either by collecting it in the world or buying it from a shop. Return to her when you get enough to start another lesson on conjuration. She will have you conjure 5 wall and floor decorations. Speak with the professor again to take on alteration. You’ll then need to alter the style, color and size of an item. Following that, you’ll then have to alter the balcony or floor. Once you’re satisfied with a new floor or balcony, talk with Deek to change the ambiance of the room. This will complete the quest.
Hogwarts Legacy – Interior Decorating
