Here in Michigan, I believe you need to be a Master Plumber (not just a Journeyman) to open your own business.
An established plumber around here, working alone, no payroll, no employees, no hassles except for the customers, can expect to clear $60-100,000 a year after expenses and before taxes. Is that what you want?
Of course, things may be different where you live, your mileage may vary, etc, etc, but this is one of the better ways to make a decent living without a college degree.
You'd make more as a general contractor hiring other guys to build houses, but you'd have a lot more responsibility and risk too.