Your cost basis is the cost basis of the land, plus what you paid to build the house. The cost basis of the land is probably what your parents originally paid for it, using the US$ conversion from local currency as of the date they bought the land.
Likewise, the prices you paid for materials and labor are added to the cost basis using the US$ value of the expenses on the date you paid them. You can include costs paid by other people for you, because that all goes toward improving the property. You can also include certain fees like the cost of mandatory permits and inspections. (Side note, you can't include the cost of bribes, even if that is how things are done in that part of the world.). You can't include costs you didn't pay for, like the value of your own or your family's labor.
If you are audited, the IRS does not have to award anything you can't prove with reliable records. What is a reliable record? Certainly invoices and receipts. If you have good, thorough records of what you paid (like a spreadsheet with name, date, amount and purpose, for all your expenses), that would give the auditor confidence in your overall meticulous nature and record keeping, they can overlook one or two missing receipts. If your records are sloppy and incomplete, or if it looks like you tried to add items from memory the night before the audit, they aren't required to believe you. So be reasonably careful and thorough if you can.
