When I get involved in residential projects, it is generally when the project goes beyond the scope of the builder.  Factors that come into play are overall size, and specialty items such as basketball/racquetball courts, swimming pools, large aquariums, stone roofs, spiral/curved staircases, long span rooms, domed or arched ceilings etc. etc..  These items typically require construction and design beyond the scope of a typical builder, and that is where I come in.