This took me a little while to find. If you are facing the house look to the left and you will see a little graveyard. Walk over there and you will see a statue of a woman. Stand to the side of this statue and you will see that you can interact with the statue. Push the statue and it will shine light onto one of the grave stones and you will see that the key is hanging on the back of the tomb stone / cross.
In future it's worth remembering that an icon will appear at the top right of the screen when you are able to interact with something. Most of the time it's not that much help, but if your stuck it can come in seriously handy!