You can unlock this just over halfway through the story.
Once you have the ability to pick locks, head to where you first entered the mansion. Go North into the room with a balcony and a massive window. Smash the glass at the North end of this room, and head out into the gardens.
You don't actually need to pick locks to get this achievement. However, the previous area of the mansion didn't have access to the gardens, so this is the best point of reference.