A full PVC shed constructed on open terrace in multi flat society below my balcony

Dear Sir, I have recently purchased a flat in a newly built multi apartment and officially completed the sale deed recently. A flat owner staying below on 2nd floor flat has constructed PVC Shed 20 days back on lease deed shown approved site-plan "open terrace (10 feet by 13 feet)", below my balcony, at height of my balcony. He claims that he has given money to builder for doing so. Tbe flat owner says that builder has given in writing that you can construct shed but can not construct cement construction and has taken money for it. For this PVC shed construction on open terrace, which is shown open in site plan with sale lease deed, I am not informed by builder while I had booked flat in beginning of project. Due to this, it has obstructed my view below, also this ruins the look of my flat when looked from outside. in addition the PVC shed roof accumulates dust, dirt, bird droppings which gives a bad look from my balcony and one window. Mutual talks have failed and I want to perceive legal action but dont know whom and where to approach. any online complaint redressal site?. Please advise