Thief: Beauty Within quest: how to enter the building for the quest?

I just started playing Thief on the PS3 and got the quest Beauty Within to steal the hand mirror for Basso. The map shows the target in Stonemarket, in a building beside the clock tower, and I've found a window on the back side of that building which I assume is the way in for the quest. However, I can't find a way to reach that window. There is a "rafter" that juts out beneath the window, which you obviously must jump onto somehow, however the rooftop pathways and ledges in the area don't take you near enough to jump onto it. So that makes me think you must have to drop down from the rooftops above, and I can't find a way to them.

I've attached a photo of the map showing my location at the window as the arrow, and the objective as the circle just above me to the right, inside the building.

Any tips for getting into the building for this quest?!