Long term capital gain.

How will you substantiate the cost incurred without the bills?

You are claiming that there was expenditure then burden of proof is also on you.

Even if you go in appeal the following provision of the Indian Evidence Act will trouble you

S101. Whoever desires any Court to give judgment as to any legal right or liability dependent on the existence of facts which he asserts, must prove that those facts exist.

(Even after applying S. 50C there is loss?)

Practically what is done in such cases is .......... something which I do not want to say. But during the proceedings AO can definitely ask how you computed capital gain and will ask to substantiate your claim. The amount is not trivial

The proof of actual construction could be permission from MCD , property tax return , original registry and the current structure. But none of it will substantiate the expenditure.