Capital gains on underconstruction property

If this property was held as investment then short term capital gain would be attracfted and hence you need to pay 15% STCG. If it is held as stock in trade then profit will be taxable as per applicable slab.

I have a follow up question on this:

1. Few other experts are suggesting that 15% is only for shares but not for "rights of property" as it is under construction. In this case, a right to property is termed as capital asset & hence normal tax slab will apply for STCG.

1. How to arrive the cost of acquisition? Can I include interest paid on borrowed capital for purchase of under construction property?

2. How to arrive fair market value? It is in new gurgaon sector86 for which circle rate is not available.

Please clarify!