Short answer: yes, an urban planter box can damage a balcony floor or leave stains if water, soil, fertilizer, rust, or weight sit against the surface for long periods. But in most rental balconies, the risk is manageable with a few simple precautions.
First, identify your balcony floor. Concrete can discolor or develop white efflorescence from trapped moisture. Tile and grout can absorb dirty water and stain. Wood or composite decking can warp, rot, or grow mold. Painted surfaces can blister, and metal can rust if water pools.
The biggest culprits are drainage and contact. A planter that drains directly onto the floor will likely leave a ring, especially if the saucer stays wet. Terracotta can wick moisture and leave mineral stains. Metal planters can rust. Cheap plastic can become brittle and crack. Fertilizer and potting mix can also stain.
To protect a rented balcony, use a sealed, self-watering planter or a lined pot. Always place it on a waterproof tray or boot mat with a raised grid, then add pot feet or a plant caddy so the planter never touches the floor directly. Empty the tray after rain or watering. Avoid overwatering, and never let the planter sit in standing water. If the balcony is wooden, use a rubber-backed mat and check underneath monthly. For concrete or tile, a mat plus feet is usually enough. Keep fertilizers and pesticides off the floor, and wipe up spills immediately.
Also think about weight. Wet soil is heavy. Large planter boxes can stress a balcony, especially older ones. Check your lease or ask your landlord before installing anything permanent. Do not drill into the balcony floor or railing. If the balcony is windy, secure the planter so it cannot tip or slide.
If you notice a stain, clean it early. Use mild soap and water for most surfaces. For rust, remove the planter and treat the mark with a suitable cleaner for that material. For mold, dry the area and clean with a diluted vinegar or mold-safe product. When you move out, lift the planter, inspect the floor, and clean any marks so you can return the balcony in the same condition.
In short, an urban planter box does not have to damage a rental balcony floor. With a waterproof barrier, raised feet, good drainage, and regular checking, you can enjoy your garden while protecting your deposit.