Delay in possession of Flat under construction

I have purchased flat ( Maharashtra) under construction stage and paid all milestone payments. later on builder has changed the plan and constructed one more floor. builder was supposed to complete the construction by Dec,15. building is still under construction and construction work of top floor, finishing work of other floors and common work like water tank, lifts installation and others are still pending. some flats are not completed at all whereas some are near to completion. i am paying regular EMI and interest thereof. Builder has completed shops and started earning of rent and own business. i have residing in my own flat in same city but facing financial losses due to this delay. also, my daughter is a patient of autism and we are facing a lot of problem to accommodate her in my small flat. my son is also facing problem in his studies due to my existing small size of flat. this loss can not be calculated in value. basic purpose of buying this big size flat was to accommodate my daughter in proper way so that my son can concentrate on his studies. please tell me the options available for me.