Modelling a Georgian bay with single sash window

[ThomasWilder]

Hello,

I am trying to find the best way to model a Georgian window, with an angled inset into the wall's thickness, and a window along the inner edge (see the attached image). Note that this isn't a box window - there is no "bump out" - the window is set into the wall. I thought about using a wall niche, but I do not know how to create the angle. 

Any suggestions would be greatly appreciated.

Software version: Interiors X13