That’s partly true – if you want to add a balcony, then it would be at your expense. Why should the OC pay for it?
So what you would have to do is seek OC approval for you to add a balcony which you are paying for. If the OC gave its approval, you would need to put a special by-law in place making you responsible for the balcony. To that extent it wouldn’t be common property.
The resolution to approve that by-law would be a special resolution, i.e. 75%.