If water from your planter is dripping onto the balcony below, treat it as a drainage and waterproofing problem, not just an overwatering problem. Start by emptying the planter and inspecting it carefully. Look for open drainage holes, cracked seams, overflow points, and any gap between the planter and the floor. If the box has drainage holes and no tray, water will eventually reach the neighbor. Also check whether water is running down the outside of the planter or seeping through the balcony slab. If the slab itself is leaking, stop using the planter and contact the building manager, because that is a structural waterproofing issue rather than a simple planter problem.
The most reliable fix is a two-part system: a waterproof liner and a drip tray. Line the inside of the planter with a flexible pond liner or heavy-duty waterproof membrane, bring it up the sides, and seal any seams with a waterproof sealant suitable for planters. Do not seal the drainage holes in a way that traps water and drowns the roots. Instead, create controlled drainage: add a drainage layer of gravel or lightweight hydroton, cover it with filter fabric, and use a tray or reservoir underneath the planter to catch overflow. If the planter sits directly on the balcony floor, place a waterproof mat under the tray to protect the surface.
Then reduce the amount of water that escapes. Use a saucer or drip tray large enough for the planter, empty it after heavy rain or watering, and check it weekly. Water slowly and stop when water begins to appear in the tray. A moisture meter or self-watering planter can help prevent overwatering. Keep the planter slightly elevated with pot feet so air can circulate and you can spot leaks early. If the planter is heavy, do not lift it alone; ask for help and check the balcony’s weight limits.
Finally, test the setup before planting. Fill the planter with water, wait 24 hours, and inspect underneath. If any drip appears, add another waterproof layer or adjust the tray. Talk to your neighbor politely, explain the fix, and ask them to tell you if they see moisture again. That combination—waterproof lining, controlled drainage, a drip tray, and smarter watering—should stop the leak and protect both balconies.