If you have sufficient proofs for the construction expenses, it shall be allowable as deduction.
In case you don't have sufficient proofs, a valuation report can be obtained in this reference.
Further, the cost of acquisition and cost of improvement (construction) shall be indexed till the year of sale.
(We may assist you with the capital gain calculations)
Sales consideration less (indexed COA and indexed COI) shall be the long term capital gains.
You may further invest the capital gain in a residential house property or section 54 EC eligible bonds to exempt the capital gain taxes.