As a true owner builder, you personally do any amount of the labor you choose, usually most of it. The figures below are based on the owner doing all of the labor. Costs are strictly for materials. This, of course, is the least expensive way to go. However, the property owner has to devote a tremendous amount of time to the project. Lending institutions do not usually encourage this format.
You could conceivably save another 10% - 20% because now all costs are for materials or incidental labor only.
You could also spend an additional 30%-40% because of poor scheduling. Many times a project is undertaken only to find out halfway through it that you cannot finish it in the timeframe you had scheduled. This can delay a portion of the construction phase which will in turn delay your building project.