This depends on the scene...I think the easiest way is to just texture the building so that it looks like light is coming out of the windows. This really only works if for instance it is a cityscape or a far away shot with no real reflections and such around it.
Now, if its a close up shot and you want the light to reflect off the trees, and you have actually modelled out the room, and windows, etc. Then I would suggest putting point lights where you think lights would be in the room, ie where chandelers would be, candles, or ceiling lights, etc. Then adjust the intensity and placement so that it comes out of the windows nicely.