In most cases, no—urban planter boxes do not absolutely need a liner before you add soil, but many do benefit from one. The right choice depends on the material of the box, what you plan to grow, where it sits, and how much planter box drainage you can provide.
Think of urban planter box lining as three different jobs: waterproofing, separation, and drainage. A waterproof liner protects wood, metal, or the surface under the box from constant moisture. A separation fabric or geotextile keeps soil from washing out of drainage holes. A gravel drainage layer is usually not necessary and can sometimes make things worse by raising the water table inside the box.
If your planter is made from rot-resistant, untreated wood such as cedar or cypress, and it has plenty of drainage holes, you can often skip a waterproof liner. The wood will age and may eventually rot, but it will not necessarily harm your plants. For edible crops, however, avoid old unknown wood, creosote-treated timber, and older pressure-treated woods. In those cases, use a food-safe liner or place a separate inner pot inside the box.
Metal planter boxes often need a liner more than wooden ones. Metal can rust, heat up in the sun, and react with wet soil. A heavy-duty HDPE liner, EPDM pond liner, or food-safe plastic can protect the metal and reduce soil contact. Make sure the liner does not block drainage. If you line the whole box like a bathtub, drill or cut drainage holes in the liner as well as the box, or leave the bottom partly open.
Concrete and brick planters can leach lime or salts over time, especially when new. A liner can help protect plants and make the soil easier to manage. It also keeps moisture from staining balconies, patios, and rooftops.
For almost any urban planter box, a geotextile fabric over the drainage holes is a smart addition. It allows water to pass while holding soil in place. Do not confuse geotextile with a waterproof raised bed liner; landscape fabric is not a substitute for a waterproof membrane. Also, avoid using a solid plastic sheet without holes. That traps water, leads to root rot, and can make the soil sour.
So, do urban planter boxes need lining before adding soil? Not always. Line when you need to protect the box, the surface below, or your plants from contaminants, rust, or rapid drying. Skip a full liner when the box is already safe, rot-resistant, and well drained. If you do line, choose a durable, food-safe material where possible, keep drainage working, and remember that the best planter box is not the one that holds water—it is the one that lets excess water escape while keeping enough moisture for roots.