An average residential balcony does not have one simple maximum weight. In many modern residential buildings, the balcony is designed for a distributed live load of roughly 1.5–3 kN/m² (about 30–60 psf), but older buildings, small cantilevered balconies, and locally regulated designs can be different. That load is not a target you should fill to; it must also cover people, furniture, pots, soil, water, and any permanent finishes. If a balcony has 4 m² of usable area and is designed for 2.5 kN/m², the total live-load allowance is about 1,000 kg, but that is a rough illustration, not a safe limit for planters.
For a few urban planter boxes, the added weight is often manageable if the boxes are small to medium, spread out, and the balcony is in good condition. The hidden issue is wet weight. A 50-litre planter can weigh roughly 60–100 kg after watering, depending on the growing medium, drainage layer, and pot. Three such boxes could add 200–300 kg, which is significant on a small balcony. Permanent built-in planters should be treated as dead load and checked more carefully.
Keep total planter weight well below the balcony’s design capacity, use lightweight soil mixes, ensure drainage, and avoid clustering heavy boxes in one spot. If you are planning large planters, live in an older building, have a cantilevered or visibly cracked balcony, or are unsure of the design load, ask a structural engineer or the building manager before loading it. For a few ordinary planter boxes on a sound modern balcony, the risk is usually low, but the exact maximum depends on the balcony’s structural design and local code.