No—you generally do not need to line the bottom of an urban planter box with rocks. In most containers, a layer of gravel or stones does not improve drainage. It can actually make drainage worse and reduce the space available for roots. For good urban planter box drainage, what matters far more is that your planter has drainage holes, uses a suitable potting mix, and is positioned so excess water can escape freely.
The common idea behind rocks in the planter bottom is that stones create a drainage layer and water flows through them. But in a container, water moving down through fine potting mix reaches the boundary with coarse rock. Capillary action holds water in the fine mix above that boundary until it is nearly saturated; this is called a perched water table. The result can be a wet zone around the bottom of the roots, even though the planter has drainage holes. A rock layer also adds weight—a serious issue for balcony planter weight limits and rooftop gardens—and steals root space.
Instead, cover the planter box drainage holes with a mesh screen, landscape fabric, or a piece of window screen. This keeps potting mix from washing out while letting water pass. Use a lightweight, free-draining mix with perlite, pine bark, or coarse compost. Avoid garden soil in containers because it is heavy and compacts. Container gardening drainage improves when the mix is airy, the holes are clear, and the box is raised slightly off the ground with pot feet. Water until water runs from the holes, and empty any saucer so the box does not sit in standing water.
If your urban planter box has no drainage holes, rocks are not a substitute. Drill holes if the material allows. If you cannot drill—for example, a decorative balcony container—use it as a cachepot with a nursery pot inside that has drainage, or choose a self-watering planter with a proper reservoir. For very large raised planter boxes, ensure the base has openings and is not sealed; a gravel layer is usually unnecessary unless a specific engineered drainage system calls for it.
You might use a thin layer of coarse material at the very bottom for a specialized reason—such as covering a large open gap or stabilizing a very tall, lightweight planter—but do not rely on rocks for drainage. In short: skip the rocks, check the drainage holes, add mesh, use quality potting mix, and manage the water that comes out. Your plants—and your balcony structure—will be better off.