(1) Except in case of fraud, failure to file a return, failure to comply with the withholding provisions of this ordinance, or omission of substantial portions of income subject to the tax, an additional assessment shall not be made after 4 years from the date the return was due, including extensions, or from the date the return was filed, or the tax was paid, whichever is later. An omission of more than 25% of gross income is considered a substantial omission of income. Under this section a declaration of estimated tax is not considered a return.

    (2) If the federal internal revenue service and a taxpayer execute a waiver of the federal statute of limitations, as to a taxable year, the expiration of the period within which an additional assessment may be made by the administrator or the department or a claim for refund filed by the taxpayer for such taxable year for city income tax purposes shall be 6 months from the date of expiration of the waiver.