Short answer: not always, but in many urban planter boxes, lining is a smart step before you add soil. Whether you need it depends on the planter material, how long you want it to last, and whether you are growing edibles.
If your planter is made of untreated wood, a liner can help slow down rot and keep soil from sitting directly against the wood. Use a food-safe waterproof liner if you are growing vegetables or herbs, and make sure water can still drain freely. Punch drainage holes in the liner that line up with the planter’s drainage holes, and consider adding a geotextile fabric layer to stop soil from washing out.
Metal planter boxes often benefit from a liner too. It can reduce rust, protect roots from heat buildup, and separate the soil from any coating you are unsure about. Concrete planters may also need sealing or lining to reduce moisture loss and limit leaching, especially with new concrete. Plastic planter boxes usually do not need a full liner, but they still need good drainage and a fabric barrier over the drainage holes.
The most important rule is never to create a sealed container. Urban planter boxes need drainage so roots do not sit in water. If you use a liner, keep it loose enough to allow water to move toward the drainage holes, and avoid trapping a waterlogged layer at the bottom. For food crops, choose food-safe materials such as HDPE, EPDM, or food-grade plastic rather than unknown recycled plastics or treated materials.
So, do you need to line an urban planter box before adding soil? Not always, but it is often worth doing for wood, metal, and concrete planters. If you have a sturdy plastic planter with working drainage, you can usually skip the liner. In every case, put drainage and food safety first, then add your soil and plants.