Short answer: not always. A liner is a tool, not a rule. Whether your urban planter box needs one before you add soil depends on what the box is made of, where it sits, how it drains, and what you plan to grow.
If your planter is wooden, a liner can be helpful. Untreated or lightly treated wood will slowly rot when it stays in contact with moist soil. A breathable geotextile or a food-safe waterproof liner on the sides can keep soil away from the wood while still allowing water to move down and out through drainage holes. Do not seal the bottom completely. If water cannot escape, roots will suffocate, and the box becomes a swamp.
Metal planter boxes often benefit from a liner too. Galvanized steel can leach small amounts of zinc over time, and plain steel can rust. A liner separates soil from metal and protects the finish. Use a non-toxic liner if you are growing herbs, vegetables, or fruit. If the metal box already has a powder coating or food-grade finish, a liner may be optional, but it can still make cleaning and replanting easier.
Concrete and stone planters may need a liner for a different reason. Lime and minerals can slowly raise soil pH, and moisture can stain or crack the surface. A waterproof liner can protect the planter, but again, drainage is essential. A false bottom or drainage layer under the soil is better than a fully sealed bag of soil.
For plastic planter boxes, a liner is usually unnecessary. Plastic does not rot or rust, and it does not react much with soil. The main exception is if the plastic is thin, cracked, or not food-safe. In that case, a liner can add protection, but it may be simpler to replace the box.
The most common liner for urban planter boxes is geotextile fabric, often called landscape fabric. It is permeable, so water passes through while soil stays in. Place it over drainage holes and up the sides if you want to protect wood or metal. Avoid solid plastic sheeting across the bottom unless you have designed a drainage reservoir. Solid plastic can block drainage and lead to root rot.
If your planter sits on a balcony, rooftop, or indoor floor, you may need two kinds of protection. One is inside the box: a geotextile or liner to manage soil and moisture. The other is outside or under the box: a waterproof tray or membrane to protect the building surface. Never rely on the planter liner alone to protect a roof or balcony. Check local rules and building requirements, especially on rooftops.
Do you need a liner before adding soil? In many urban planter boxes, yes, but not always. Use a permeable liner if the goal is to keep soil in and let water out. Use a waterproof liner on the sides if the goal is to protect wood, metal, or concrete. Skip the liner if the box is plastic, has good drainage, and does not need surface protection. The key is to match the liner to the problem you are solving.
Before adding soil, check drainage first. Add a layer of gravel or a false bottom only if it improves drainage in your specific design; in many containers, a false bottom can actually raise the water table. Fill with a quality potting mix, not garden soil, because potting mix is lighter and drains better. Then plant, water gently, and watch how quickly water exits the box. If water pools on the surface, fix drainage before adding more soil or plants.
In short, urban planter boxes do not always require a liner, but they often benefit from one. The right liner protects the box, keeps soil where it belongs, and supports healthy roots. The wrong liner traps water and causes problems. Choose based on material, drainage, location, and what you are growing.