The short answer is: not always. An urban planter box needs a saucer underneath mainly when it sits on a surface you want to protect—such as a balcony floor, rooftop deck, windowsill, or indoor floor—or when neighbors below could be bothered by dripping water. A saucer catches excess water that drains out of the holes, helping prevent stains, water damage, slippery tiles, and complaints.
However, a saucer is not automatically better. If water stays in the saucer and the planter sits directly in it, the soil can remain waterlogged, which may lead to root rot, fungus gnats, and poor plant health. That is especially true for herbs, succulents, and many vegetables that dislike wet feet. If your planter box is outdoors in a garden bed or on soil, it usually does not need a saucer; let the water drain freely into the ground.
For most urban balconies and terraces, the best setup is a planter box with drainage holes, a saucer or drip tray, and small pot feet or a riser to keep the box out of standing water. Empty the saucer after heavy rain or watering. If the planter has a built-in reservoir or self-watering system, you may not need a separate saucer, but you should still check for overflow.
So, do you need one? Use a saucer if you want to catch water and protect surfaces. Skip it if drainage is free and standing water is not an issue. The key is not just catching water—it is making sure your plants never sit in it.