Amount received from the builder for under construction property

I want to purchase a flat and I asked a builder & I paid lump sum amount for that flat in Jan 2017..he made a promise that he will deliver the flat in 2019 - 2020 . as per agreement builder is paying incentive for such 3 years. my doubt is such incentive to be show in 'income from house property' or 'income from other source'
whether it is liable for tax or not.