It is the chase that causes falls, not clumsiness
Cats are superb athletes, which is exactly why owners underestimate this risk. A cat on a balcony is not strolling near an edge she fears; she is hunting. A fly, a pigeon landing on the rail, a leaf blowing past — any of these can trigger a full-focus lunge in an animal whose attention has narrowed to a single point, and the landing surface she calculated for is the rail, not the air beyond it. Cats also slip: a startle, a wet rail, a stretch rolled too far in sleep. Veterinary clinics see the results so reliably that the pattern has its own name, high-rise syndrome, and the myth that cats always land safely does them real harm — many survive falls and still arrive with broken jaws, limbs and internal injuries.
Enclose the whole space, not just the railing
The only trustworthy answer is to turn the balcony into a sealed outdoor room. Fit netting or rigid mesh made for cats — fine enough that a head cannot push through, strong enough to take her full weight landing against it at speed, and fixed to a frame or the balcony structure rather than hooked loosely. Run it from floor to the balcony above with no gap at the top, because a cat who cannot go through will go up and over any partial barrier. Check the sides where your balcony meets the neighbour's, the gap beneath the door, and any space between rail and wall wide enough for a determined cat, which is roughly anywhere her head fits. Transparent cat netting exists for buildings where appearance matters, and in many blocks it passes management rules where solid panels would not — ask first rather than fitting something you must remove.
Habits that close the gaps netting cannot
Enclosure is hardware; the failures after that are human. Make it a house rule that the balcony door is opened only when the netting has been checked and the cat is accounted for — never propped open for air while she wanders the flat. Inspect the mesh regularly for chewed corners, sun-rotted fixings and the slow sag that opens a gap at the base, and after any storm. Keep chairs, storage boxes and plant stands away from the edges so she is not launching from furniture toward the rail. Choose balcony plants with cats in mind, since an outdoor space she uses daily should not include lilies or other plants poisonous to her, and never use the balcony for fireworks-adjacent celebrations or anything involving sudden bangs at close range.
Harnesses, leads and supervised outings: the honest limits
A harness is not a substitute for enclosure, and treating it as one is how accidents happen. A startled cat can back out of many harnesses in a second, a lead long enough to let her explore is long enough to let her go over the rail and be left hanging, and your attention will wander eventually — that is what attention does. Carrying her out in your arms for a look is the riskiest option of all. If the balcony cannot be enclosed, the comfortable truth is that a window perch inside a properly screened, barely-opened window gives her the same sun, smells and bird-watching with none of the exposure, and most cats accept the trade without a murmur.
Even with everything done right, flats have visitors, cleaners and open doors, so keep her identification current as the last line of defence. Should she somehow get out of the flat and vanish into the building or the street below, the Lost Pet Alert System on KinsLe is included in every profile at no charge, and a person who spots her can send you a message through her alert — the minutes that matter most are the first ones, while she is still close.