Is obtained in Chapter 3 when talking to Silas the Caretaker (the conversation after he threatens you with the axe).
Technically missable, as some failed skill checks or bad conversation may cause you to have to fetch the key from the cabin. Fortunately, there is a conversation path where you don't need any skill checks.
First, on your way to the manner before meeting Silas, make sure you take the left path to the graveyard and inspect Sarah Hawkins grave -- this will unlock a conversation item that will get Silas to cooperate. From the grave, continue on the level until you encounter Silas.
During the conversation, answer as the following:
-- I work for Stephen Webster (no requirements needed for this choice)
-- Sarah Hawkins was important to you (must have inspected Sarah Hawkins grave)
He'll give you the key to the mansion after this, and achievement unlocked!