How to calculate Cost Basis on a Land Bank purchase

[Rene Brodowsky]

We are within 5 years of starting to sell off our investment properties. Two of them were Land Bank purchases. (closed for 1,500 and 3,700, respectively) We did all the improvements ourselves (minus the roof work) Am I using those closing costs and adding all improvements to calculate our cost basis?  (Or will IRS use some fair market value number??)