You should make conservative estimates of the improvements and add it to the cost basis. The IRS is unlikely to complain, given the time period involved, as long as they are reasonable.
Of course, if the gain, without the adjustment to cost basis, will be less than $250,000 ($500,000 if filing jointly), then there is no need to estimate at all because the entire gain will be excludable. It won't matter if the gain is $1 or $249,999.