If the ownership of the plot purchased and the newly construed apartments are in the name of your uncle only, there is no capital gains, till such he actually sells the property.
Now that he has sold one floor, he has to pay capital gains on the sale of such property. However, the buyer will not get ownership of the entire plot and instead he may just get undivided share in the land.
Now the cost of acquisition will have 2 components - the cost of undivided share of land proportionate to the floor being sold and the cost of construction. As the cost of construction is incurred only in 2016, it will be short term capital gain and long term capital gain in the case of land, i.e., on the undivided share of the land.
The exact tax liability can be worked out on review of the documents executed. Otherwise, the opinions may not be useful to you.