Section 35 ILCS 5/903 - Assessment(a) In general. (1) Returns. The amount of tax which is shown to be due on the return shall be deemed assessed on the date of filing of the return (including any amended returns showing an increase of tax). In the event that the amount of tax is understated on the taxpayer's return due to a mathematical error, the Department shall notify the taxpayer that the amount of tax in excess of that shown on the return is due and has been assessed. Such notice of additional tax due shall be issued no later than 3 years after the date the return was filed. Such notice of additional tax due shall not be considered a notice of deficiency nor shall the taxpayer have any right of protest. In the case of a return properly filed without the computation of the tax, the tax computed by the Department shall be deemed to be assessed on the date when payment is due.(2) Notice of deficiency. If a notice of deficiency has been issued, the amount of the deficiency shall be deemed assessed on the date provided in section 904(d) if no protest is filed; or, if a protest is filed, then upon the date when the decision of the Department becomes final.(3) Federal change. If an amended return or report is filed with the Department pursuant to section 506(b), any deficiency in tax under this Act resulting therefrom shall be deemed to be assessed on the date of filing such report or amended return and such assessment shall be timely notwithstanding any other provisions of this Act.(4) Payments. Any amount paid as tax or in respect of tax paid under this Act, other than amounts withheld or paid as estimated tax under Articles 7 or 8, shall be deemed assessed upon the date of receipt of payment, notwithstanding any other provisions of this Act.(b) Limitations on assessment. No deficiency shall be assessed with respect to a taxable year for which a return was filed unless a notice of deficiency for such year was issued not later than the date prescribed in section 905.