Trespassing by a neighbor living downstairs

I stay in a DDA flat in Saket, Delhi. A property dealer neighbor lives downstairs and has installed an iron staircase in his backyard from ground floor to a vacant area that adjoins my flat on the first floor and is accessible to me. He says it's his terrace as he constructed the rooms below and we say it's our level and it's trespassing and breach of our security and privacy. Throughout the colony, there is similar encroachment/extensions by the ground floor residents and mostly they also allow residents of above floors to construct rooms on top of their extended rooms. However, my bully neighbor, who's living here since decades (while my family moved just 3 years back), refuses to understand and accesses this area at my level frequently without informing us, which makes us anxious and insecure. Can I stop his access and cover this area going through the legal route?