From the Life Safety Code as an [Extract]
3.3.25.4*
Building, Existing.
A building erected or officially authorized prior to the effective date of the adoption of this edition of the Code by the agency or jurisdiction.
*FPN: With respect to judging whether a building should be considered existing, the deciding factor is not when the building was designed or when construction started but, rather, the date plans were approved for construction by the appropriate authority having jurisdiction.