Capital gain on house property

I live in a residential flat which is the only property in my name. Planing to buy a residential plot worth 28-30 lacs taking bank loan only for plot, before March 2017 and get it registered in my name. Further, I wish to start construction of residential house on that plot by March 2017 either giving construction contract with material to a third party with agreement or only labour contract to a third party. I may or may not opt for housing construction loan. Construction agreement will be done before March 2017. After about a year ie nearly completion of the house, I plan to sale my existing flat and expect net capital gain considering indexation, as about 15-18 lacs. Sale agreement will be either in FY 17-18 or in FY 18-19 beginning. Can I claim construction cost of house to save capital gain tax on flat which I sold after or nealy completion of the construction? I do not want to sell flat before completing house. Do I have to restrict flat sale agreement in a particular FY? Kindly guide how to plan all above so that I can legally save capital gain tax.