Start by finding out whether the drip is overflow, a leaking drain, condensation, or rain runoff. On a balcony, the cleanest fix is usually a two-part system: a waterproof liner inside the planter and a catch tray or drain hose underneath that takes excess water to a bucket or approved drain. Choose a tray with raised edges that is wider than the planter, and set the planter on feet so you can inspect and empty the tray. If the tray has a drain, connect a flexible hose and secure it so it cannot flap over the edge. Do not simply drill a new hole through the balcony wall or facade; check your lease and building rules first.
Line the planter with pond liner, EPDM, or heavy-duty plastic, sealing around the drain with waterproof sealant. Give the liner a slight slope toward the drain, and use a bulkhead fitting with a hose if possible. A self-watering insert or reservoir can also reduce free drainage, but it still needs an overflow tube aimed into a container. Replace loose potting mix with a mix that holds some moisture but drains well, and add mulch to slow evaporation. Water slowly with a drip irrigation timer or a watering can, and check the soil before watering rather than watering on a fixed schedule.
Keep the system maintained: clear leaves from the drain, check the hose after heavy rain, empty the bucket before it overflows, and test the planter with a measured amount of water. If the planter is against a wall, keep a small gap for airflow and inspect for hidden dampness. Talk to your neighbor early, explain what you are changing, and ask them to tell you if they still see drips. If the leak may affect the building structure or a shared balcony, involve your landlord or building management. The goal is not just to catch water but to control it at the source so nothing reaches the neighbor’s balcony.