Capital gain - cost of improvement

Sir/Ma'am,

I am computing capital gain where assesse has stated cost of improvement w.e.f 1981 on his house property but does not have any receipts for the same.

All these COI are before FY 2007.

Please guide me, Its an Urgent Matter

1. Whether Assessing Officer can ask for the receipts of improvement or such COI expense being beyond four years from the end of the relevant Assessment Year will not be asked for

2. Whther such Cost of improvement can be challenged by Assessing Officer? If Yes, then what to do.

Thank You