Photo Compositing

Photo compositing can really save time by not having to focus on modelling/rendering the environment. It also allows the viewer to directly relate the proposed and the existing since they share the same background and angle. 

Photo Match in SketchUp vastly simplifies the process if you've got a good photo with good X and Y axis references to use. A simple tutorial can be found on SketchUpArtists.org , an invaluable resource: Photo Match Tutorial