The golden key is inside but the red gem opens the door out. I tried letting it roll down the stairs but it doesn’t open the barrier on the door if I am not holding it. How are you supposed to get the key outside of the house?
You will need to go down the hall and into the room where you see the small owl statue. You can throw the key out of this window and if you head back to the center area you will see the key lying on the ground. You just need to make it big again and you can use it on the door.