Yes, in most cases an urban planter box should have drainage holes—but the drainage needs to be managed carefully so it does not leak onto the neighbor below. Drainage holes allow excess water to escape, which helps prevent waterlogged soil, root rot, fungal problems, and unhealthy plants. Without drainage, especially outdoors where rain can enter the box, water can collect at the bottom and turn the soil sour and oxygen-poor. That said, a planter box on a balcony, terrace, or rooftop directly above someone else should not simply drain freely through the floor or over the edge.
The practical solution is to use drainage holes plus a way to capture and redirect the runoff. A sturdy drip tray or saucer under the planter can collect excess water, as long as you empty it before it overflows. For larger urban planter boxes, use an integrated drainage tray, a false bottom, or a self-watering reservoir designed so overflow is contained. You can also line the inside of the box with waterproofing and create a controlled drainage layer that leads to a small overflow outlet, then direct that outlet into a tray or safe drain—not onto the balcony below. Elevate the box slightly so air can circulate and so you can inspect for leaks.
If the planter is sealed with no drainage holes, you must water very carefully and use a sub-irrigation or reservoir system, but this is riskier outdoors because rainwater has nowhere to go. Never rely only on a layer of stones to solve drainage. Also check your lease, building rules, and waterproofing requirements before installing a heavy or permanently watered planter above a neighbor. In short: drainage holes are usually necessary, but they should be paired with a tray, liner, reservoir, or controlled overflow so the water goes where you intend—not through your neighbor’s ceiling.