Property indexation

A person bought a bouse in 2004 under construction linked plan and every year he paid some amount and in 2009 he finally got the property and in 2018 he sold it. Now my query is to what price the indexation will be applied as payment was distributed in 5 years. (Total paid 20L, each 5 years paid 4L) So how will the base price and year be determined for indexation?