Diligently. Your cost basis is the cost of the land, materials, and labor you paid for. Plus other required legal costs such as for permits, inspections and surveys.
If you don't have your records, you have a problem. If audited, the IRS doesn't have to give you credit for anything you can't prove.
Taxpayers should always save records related to the purchase or construction of a home for the entire time they own it plus at least seven years after they sell.
