One of the most important skills for any do-it-yourselfer is to know when not to. If you have a really big project in mind, like adding a room or a project that involves skills (and tools) outside of your knowledge, turning to professionals is a good choice.
Now you enter the realm of contractors and subcontractors. What's the difference? Think of it like this. If you hire an event planner for a wedding, golden anniversary or other celebration, you don't expect him or her to bake the cake, cook the meal, grow the flowers or play the music. You pay the planner to use his or her knowledge of the industry and organizational skills to bring together the best baker, caterer, florist and string quartet in a coordinated effort to make your event successful. The event planner is the contractor, and the people in his or her rolodex are trusted subcontractors.
General contractors are the "big picture" thinkers. They work with you through the entire project, from the idea phase to the finished product. By contrast, subcontractors come and go. They specialize in certain areas of the construction process, like large equipment operation, concrete formulation, plumbing, electricity and carpentry. They enter a project to do a particular task and leave when that task is finished.