If you build the house as a business venture, it would be business income, not capital gain.
So, how do I enter the proceeds ( income)) and cost basis on my schedule C? I don’t see any links for this 1099-S besides on the investment section or main home sale .
To report your property sale on Schedule C, you would visit the Self-Employed income and expenses section in TurboTax under Wages and Income and set up a business and then enter the property sale as if it were income from a typical business. You would not enter in the Form 1099-S as such, you would just report the income listed on it as business sales. The cost of the property would be entered as purchases, a component of cost of good sold.
You would report the income as self-employment income only if you are a real estate professional. The basic requirements for that are that you perform over half of your yearly services in the real property trades and perform over 750 hours of services in those trades in the year. Here is an excerpt from IRS Pub 925:
If you don't qualify as a real estate professional, you would enter the sale as the sale of an investment property.