Tax exemption on mortgage loan used for house construction

As I understand, you can claim deduction u/s 80C for principal repayment and u/s 24b for interest liability.

My house is still under construction (70% completed), from when can I avail the tax exemption? What documents from the Co-operative bank do I need to furnish?

You can claim deduction of interest (including pre-construction interest) after completion of construction. You need to obtain interest/principal repayment statement from the bank. There is no need to upload the certificate along with the return. You can retain the same for submission to AO in future if required.