How do i capital gain on sale of house property?

This event partly comes under conversion of capital asset into stock in trade

Selling part of the property in your case its a long term property,

you can claim indexation benefits,

Selling such flats attracts long term capital gain,

You can claim exemption U/S 54 and in 54EC...

However, Such capital gain must be invest in a new property or in NHAI or RECL bonds for exemption from long term gain...

it will be treated as asset converted to stock in trade valued at current market price.

Taxable as long term capital gain at current market price

and difference between sales consideration and current market price will be treated as income form Business.

if it is not for business purpose then it will taxable as long term capital gain.