A balcony only becomes safe outdoor space once a cat cannot get over, under or through anything on it, so treat the job as building an enclosure rather than adding a barrier she can test.
Enclose the whole space with sturdy cat netting or mesh fixed to a frame that runs from floor to ceiling, and work along every edge with your hands, checking for gaps at the sides where a determined cat could push her head through. Netting must be taut and firmly anchored; a loose corner she can worry at will widen week by week. If your building restricts fixings, a freestanding framed enclosure inside the balcony line achieves the same sealed result without drilling the structure.
Checking the Enclosure Like a Cat Would
Cats test a balcony at its weakest points, not its strongest. Push on the netting at cat height, tug each fixing, and look for daylight gaps where the frame meets a wall, a drainpipe or a railing post. Check the door or gate you use to reach the balcony: it needs a latch she cannot bat open, and everyone in the household needs the habit of closing it before opening the door from inside, so she never slips out during the swap. Re-inspect monthly, because sun weakens netting and fixings work loose; a panel that felt solid in spring can flex by late summer.
Furniture placement matters as much as the mesh itself. Keep chairs, storage boxes and plant stands away from the edges, since each one is a launch pad that shortens the distance to the top of the frame. A scratching post and a lookout perch placed toward the middle of the balcony give her height and interest without helping her reach the boundary.
Making the Space Worth Staying In
A safe balcony she dislikes will still draw escape attempts, so furnish it for a cat: shade that moves with the sun, a water bowl that cannot tip, a scratching surface, and a bed in the warmest sheltered corner. Add a tray of cat grass or a sturdy planter she is allowed to chew, and rotate a toy or two so the space stays interesting across the season. Let her out for short supervised sessions at first, watch where she heads and what she tests, and fix the weak point she finds before it becomes a route.
Never leave a cat on a balcony in a harness tied to a railing: a fall in a harness can injure or strangle her, and a frightened cat can wriggle out of most harnesses. Supervision alone is not a system either, because the one lunge at a passing bird happens faster than anyone can cross the room.
Keeping Every Helper on the Same Rules
Balcony safety usually fails at the handover, not the build: one person props the gate while carrying plants out, another forgets the latch check, a third lets her out before the monthly netting inspection is done. Write the rules where everyone can see them — gate latched before the inner door opens, no furniture near the edges, netting checked on the first weekend of each month — and stick to them even when she is calm and the day is quiet. KinsLe Family Sharing gives relatives you pick access to her profile, for a relative feeding her while you are away.