Using urban planter boxes on a fire escape is generally not allowed and is strongly discouraged. Fire escapes are critical safety structures designed for emergency egress for you and your neighbors, as well as access for firefighters. Local fire and building codes almost universally prohibit obstructing this path with any objects, including planters. The added weight can stress the structure, and materials like soil and clay pots become hazardous debris in a fire or emergency. Furthermore, watering plants creates slip hazards and can accelerate rust.
Instead, consider safe alternatives for your urban garden. Look into installing secure window boxes on the interior side of your window sill, using indoor herb gardens, or participating in a community garden. Always prioritize safety by checking your local fire codes and your lease agreement before placing any items on a fire escape. Your green thumb should never compromise a vital escape route.