If plants on your building's terrace are causing water leakage into your first-floor flat, and the owner refuses to address it, several steps can be taken. You can raise the issue at the Annual General Meeting (AGM) of your society and suggest temporarily removing the plants to check for leakage. Additionally, you can file a complaint with the association, police, or local civic body, and potentially seek a mandatory injunction to have the plants removed, provided you gather sufficient evidence.