Cost of improvement -capital gain

Purchased land in 1986 and built building in that in the year 1988. sold land in the year 2023. Now for the calculation of capital gain, I've ascertained FMV of land as on 2001 for indexation. How abt building value. As per rule, Cost of improvement incurred before April 1, 2001 is never taken into consideration. so how to take building value into consideration. should I consider building value as on 2001 or ignore building value for calculation.