The short answer is: sometimes, but not automatically. An urban planter box can survive winter outdoors if it is made from a frost-resistant material, has excellent drainage, and is not sitting in waterlogged soil. The biggest risk is not cold alone; it is freeze-thaw cycles. When water inside the soil, drainage tray, or porous material freezes, it expands. That pressure can crack ceramic, terracotta, thin concrete, plastic, and composite planters, especially on balconies or rooftops where wind and refreezing are stronger.
If your planter is plastic, fiberglass, or good-quality composite, it is usually less likely to crack, though it may become brittle in extreme cold. Wood can survive but may warp, split, or rot if wet. Metal may dent or rust, and ceramic or terracotta is the most vulnerable unless it is labeled frost-proof or frost-resistant. Even a frost-proof planter can crack if water cannot drain or if the soil stays saturated.
To leave it outside safely, empty it if possible, or at least stop watering and let the soil dry somewhat. Remove saucers so they do not trap water and freeze. Keep drainage holes open. Raise the box slightly off the ground or balcony floor so water can escape and air can circulate. Move it against a sheltered wall, wrap it with burlap, bubble wrap, or blankets, and add mulch over the soil if plants remain. Avoid letting snow pile up inside the planter, because meltwater can refreeze. Check it after storms.
If you cannot move it and it is a fragile ceramic or terracotta planter, it may be safer to empty it and store it in a garage or shed. If you leave it out, accept some risk. A planter that is well-drained, frost-proof, and protected has a good chance of making it through winter without cracking. A waterlogged, thin-walled, or decorative planter has a high chance of cracking. So yes, you can leave an urban planter box outside all winter, but only if you prepare it for freeze-thaw conditions.