However if you aren't content to wait, you can always try using a trick to get up to the lower roof: From the belltower door, look down to see a large tree stump to the left of a lower roof. The Handmaid sells the key for a whopping 20,000 souls which is a lot to ask for early in the game, but you should try to get it sometime around the Undead Settlement.
As you explore Firelink Shrine, you may note the side passage out of the top floor to a locked belltower door.
