Hi Mary,
Since your strata scheme was registered on the 80’s then the balcony doors will be considered common property (unless in the unlikely scenario that it is stated otherwise on the registered strata plan).
Since the doors are common property you will need a special resolution at a general meeting for approval.
Further to this, if the owners corporation considers that any approval should be based on you (or any future lot owner) being burdened with the responsibility of the ongoing maintenance of the new doors (since they are an improvement to your lot) then a by-law will need to be approved and registered also.