If it’s on the owner’s title (on their portion of the strata plan) and not marked as common property, it’s probably the owner’s responsibility. What does the owner think? If other owners accessed that part of the property, would s/he object? If so, the owner probably considers that it is part of their lot.
What are the implications of structural instability? Are you concerned that if it falls down the owner will try to recover from the Body Corporate on the basis that it wasn’t approved by you?
You could try passing a special by law to state that additions done before a certain date and incorporated in a unit holding are henceforth considered the owner of that lot’s responsibility. That might at least draw him or her out….