For a sunny urban balcony, the best urban planter box for succulents is a wide, shallow trough with excellent drainage, made from a light-colored, heat-resistant material. Think 6 to 8 inches deep, at least 16 to 24 inches long, with multiple drainage holes and feet to keep it off the hot floor. A glazed ceramic or fiberstone trough in white, sand, or pale gray is ideal because it reflects heat, holds enough moisture without staying soggy, and looks clean in a small space. Terracotta is a strong second choice: it breathes and wicks excess water, but on a full-sun balcony it dries out fast, so choose a larger size or water more often. Avoid dark plastic, metal, and narrow, deep pots. Dark plastic and metal heat up quickly and can cook shallow succulent roots; deep pots hold wet soil around the root zone and increase rot risk.
If you want the single best pick, choose a light-colored glazed ceramic rectangular planter, 16 to 24 inches long and 6 to 8 inches deep, with drainage holes and a matching saucer. Add a gritty succulent mix, not standard potting soil. A mix of about 50 to 60 percent mineral grit, such as pumice, perlite, coarse sand, or poultry grit, and 40 to 50 percent compost works well. Top with small pebbles to keep leaves dry and reduce evaporation. Place the box where it gets morning sun and some afternoon shade if your balcony becomes an oven. Succulents love sun, but intense reflected heat from walls and pavement can scorch leaves. Rotate the planter every week or two so all sides get even light.
Self-watering planters can work, but only if they have an overflow and you use a very gritty mix. Do not keep the reservoir constantly full; succulents prefer a soak-and-dry cycle. Water deeply until it runs from the drainage holes, then wait until the soil is completely dry before watering again. In hot sunny weather, that may mean every 7 to 10 days; in cooler months, every 2 to 4 weeks or less. If your balcony is very windy, choose a heavier ceramic or concrete planter, or secure a lighter resin box. If weight is a concern, use a UV-resistant, light-colored resin trough with drainage holes, but expect faster drying than ceramic.
For a polished urban look, group succulents by water needs: echeverias, sedums, and graptopetalums in one shallow box; aloes and agaves in a deeper one if you have space. Keep the box slightly elevated on pot feet so water drains freely and air moves underneath. With the right planter, wide, shallow, light-colored, and drainable, your sunny balcony can become a low-maintenance succulent display that handles heat far better than a typical dark plastic window box.