I think I know the answer to this already but here goes.
New neighbours, re-vamped their front garden and have just planted a tree next to the boundary fence with our front garden.
Not sure what the tree is - it looks like a flowering maple similar to one in our rear garden- which grows to around 15 feet high and about 10 feet across. In time their tree will block a lot of light from our dining room window, and it's already a dark room.
It's not opposite the window, but alongside.
Don't suppose there is anything we can ever do about it- apart from cutting off branches which may overhang the fence is time.