Question? is that the right hand side [when viewed from the outside] of a Bay Window?
If so suggest you have a good look at the flat roof and any parapets on the bay window, on the inside especially, you are looking for old [possibly] damaged lead or felt roof covering, also have a good look at the junction of roof covering and parapet.
Check out the rain water outlet, ensure it is running clear, no obstructions.
So much better when people remember some manners when posting. A simple greeting goes a long way. Always goes down well when posting and asking for help.