home constructed cost basis w/o receipts

Hello,

I took out a 31 year single construction to 30-yr mortgage to build my primary residence.  I was able to take money out to pay bills for the construction, paying only interest.  I ended up with a final balance that was now a interest and primary 30 year loan.  Same Note. 

 

I have lived in it for over 3 years and know I qualify for the $500k capital gains deduction.  I didn't keep receipts well.  I have read where one way to determine my cost basis is if I had a loan to buy the house, that would be the cost basis plus improvements.  So, would I be able to use the first mortgage (construction to fixed mortgage) loan as that cost basis?  I do I have to come up with reciepts or examples of FMV for parts of the construction?

 

Thank you!