Depending on how many of the different phases of the construction you can do yourself, you'll save a bundle. A general contractor gets his cut, but he usually uses sub contractors for foundations, framing, plumbing, electric, roofing, etc. All those subs have a wage to earn, and there might even be a mark-up on the materials.
Is the house worth less because you built it yourself? No, provided it is up to same quality standards that a contractor would build.
I have a friend who claims he built a spec house using his credit cards, then when complete sold it, at 300% if what it cost him to build. Turned out to be a sweet deal.
