Yes, there are urban planter boxes—including narrow windowsill planter boxes—designed to fit a narrow windowsill. The best options are slim, low-profile containers usually 5–8 inches deep, with a width that leaves enough room for the window to open and close. Look for a slim windowsill planter made from lightweight materials such as powder-coated metal, recycled plastic, or resin, because a narrow sill may not support heavy soil and ceramic. Use window box brackets or a sill-mounted system that is securely fixed; avoid relying on a loose tray, especially on upper floors or in windy cities.
For city gardening, a self-watering planter is especially useful because small volumes of soil dry out quickly. Choose a model with a built-in reservoir, drainage holes, and a drip tray or overflow guard so water does not stain the facade or leak onto neighbors. If the sill is very shallow, consider a rail-mounted or adjustable bracket planter that hangs just outside the window rather than sitting fully on the ledge. Always measure the sill depth, check the weight limit, and make sure the box does not block ventilation or emergency access.
Plant compact herbs, lettuce, strawberries, or small flowers. Avoid tall or top-heavy plants that can catch wind. With the right slim urban planter box and safe mounting, a narrow windowsill can become a productive mini garden without taking up floor space.