The height, bulk, location and use of all buildings and parking lots in existence or for which Special Permits have been issued as of the effective date of this section are hereby declared to be in conformance with the requirements of this section, provided that if such buildings are destroyed by fire, explosion, act of God or act of public enemy to an extent exceeding 50% of their assessed value, they may be reconstructed only if the height, bulk, location and use of the building is substantially as it had previously existed, subject to approval by the Director of Planning and Zoning. The owners of such property shall document by A-2 survey or other means, the height, bulk, location and use of the building as it had previously existed.