(a) Except as otherwise provided in this chapter, a notice of proposed deficiency assessment shall be mailed to the taxpayer within three years after the return is filed. No deficiency shall be assessed or collected with respect to the year for which the return is filed unless the notice is mailed within the three-year period or the period otherwise fixed. Where, within the sixty-day period ending on the day on which the time prescribed by this chapter for mailing a notice of proposed deficiency assessment for any taxable year would otherwise expire, the commissioner receives a written document signed by a taxpayer showing that the taxpayer owes an additional amount of tax for such taxable year, the period during which a notice of proposed deficiency assessment may be mailed shall not expire before the day sixty days after the day on which the commissioner receives such document.

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(b) (1) If the taxpayer omits from Connecticut adjusted gross income, in the case of an individual, or from Connecticut taxable income, in the case of a trust or estate, an amount properly includable therein which is in excess of twenty-five per cent of the amount of Connecticut adjusted gross income or Connecticut taxable income, as the case may be, stated in the return, a notice of a proposed deficiency assessment may be mailed to the taxpayer not later than six years after the date on which the return is filed. For purposes of this subdivision, there shall not be taken into account any amount which is omitted in the return if such amount is disclosed in the return, or in a statement attached to the return, in a manner adequate to apprise the Commissioner of Revenue Services of the nature and the amount of such item.

(2) If the taxpayer omits from the Connecticut adjusted gross income derived from or connected with sources within this state, in the case of a nonresident individual or part-year resident individual, or from Connecticut taxable income derived from or connected with sources within this state, in the case of a nonresident trust or estate of part-year resident trust, an amount properly includable therein which is in excess of twenty-five per cent of the amount of Connecticut adjusted gross income derived from or connected with sources within this state or Connecticut taxable income derived from or connected with sources within this state, as the case may be, stated in the return, a notice of a proposed deficiency assessment may be mailed to the taxpayer not later than six years after the date on which the return is filed. For purposes of this subdivision, there shall not be taken into account any amount which is omitted in the return if such amount is disclosed in the return, or in a statement attached to the return, in a manner adequate to apprise the commissioner of the nature and the amount of such item.

(3) If an employer, as defined in section 12-707, omits from Connecticut wages an amount properly includable that is in excess of twenty-five per cent of the amount of Connecticut wages stated in the Connecticut withholding tax return required under section 12-707, a notice of a proposed deficiency assessment may be mailed to the employer not later than six years after the date on which the return is filed. For purposes of this subdivision, there shall not be taken into account any amount which is omitted in the return if such amount is disclosed in the return, or in a statement attached to the return, in a manner adequate to apprise the commissioner of the nature and the amount of such item.

(4) If an affected business entity, as defined in section 12-699, omits from the Connecticut adjusted gross income derived from or connected with sources within Connecticut of any member of such affected business entity an amount properly includable therein that is in excess of twenty-five per cent of the amount of Connecticut adjusted gross income derived from or connected with sources within Connecticut stated in the return filed pursuant to section 12-699 or section 12-719, a notice of a proposed deficiency assessment may be mailed to the taxpayer not later than six years after the date on which the return is filed. For purposes of this subdivision, there shall not be taken into account any amount that is omitted in the return if such amount is disclosed in the return, or in a statement attached to the return, in a manner adequate to apprise the commissioner of the nature and the amount of such item.

(c) (1) If no return is filed or if a taxpayer makes, wilfully or otherwise, a false or fraudulent return, a notice of deficiency assessment may be mailed to the taxpayer at any time.

(2) If a taxpayer wilfully attempts in any manner to defeat or evade a tax imposed by this chapter, a notice of deficiency assessment may be mailed to the taxpayer at any time.

(3) If a taxpayer fails to disclose a reportable transaction, as defined in Section 6707A of the Internal Revenue Code, on the taxpayer’s federal tax return, a notice of deficiency assessment may be mailed to the taxpayer at any time not later than six years after the return required under this chapter for the same taxable year was filed.

(d) (1) If a taxpayer fails to comply with the requirements of section 12-727 by not reporting a change or correction by the United States Internal Revenue Service or other competent authority increasing, in the case of an individual, the individual’s federal adjusted gross income or, in the case of a trust or estate, its federal taxable income, or by not reporting a change or correction which is treated in the same manner as if it were a deficiency for federal income tax purposes, or by not filing an amended return, a notice of a proposed deficiency assessment may be mailed to the taxpayer at any time. The provisions of this subdivision shall also apply if an individual’s computation of tax under Section 1341(a)(4) or (5) of the Internal Revenue Code is changed or corrected by the United States Internal Revenue Service or other competent authority, and the individual fails to comply with the requirements of section 12-727.

(2) If a taxpayer fails to comply with the requirements of subsection (b) of section 12-704 by not reporting a change or correction by tax officers or other competent authority of another jurisdiction affecting the amount of tax of such other jurisdiction that the taxpayer is finally required to pay, or by not filing an amended return, a notice of a proposed deficiency assessment may be mailed to the taxpayer at any time.

(e) (1) If the taxpayer, pursuant to section 12-727, reports a change or correction by the United States Internal Revenue Service or other competent authority increasing, in the case of an individual, the individual’s federal adjusted gross income or, in the case of a trust or estate, its federal taxable income or reports a change or correction which is treated in the same manner as if it were a deficiency for federal income tax purposes, or files an amended return, the assessment, if not deemed to have been made upon the filing of the report or amended return, may be made at any time not later than three years after such report or amended return is filed. The provisions of this subdivision shall also apply if an individual’s computation of tax under Section 1341(a)(4) or (5) of the Internal Revenue Code is changed or corrected by the United States Internal Revenue Service or other competent authority, and the individual, pursuant to section 12-727, reports the change or correction.

(2) If the taxpayer, pursuant to subsection (b) of section 12-704, reports a change or correction by tax officers or other competent authority of another jurisdiction affecting the amount of tax of such other jurisdiction that the taxpayer is finally required to pay, or files an amended return, the assessment, if not deemed to have been made upon the filing of the report or amended return, may be made not later than three years after such report or amended return is filed.

(f) Where, before the expiration of the time prescribed in this section for the assessment of a deficiency, both the commissioner and the taxpayer shall have consented in writing to its assessment after such time, the deficiency may be assessed at any time prior to the expiration of the period agreed upon. The period so agreed upon may be extended by a subsequent agreement in writing made before the expiration of the period previously agreed upon and the commissioner may, in such a case, waive the statute of limitations against a claim for refund by such taxpayer.

(g) For purposes of this section an income tax return filed before the last day prescribed by law or by any regulation adopted pursuant to law for the filing thereof, determined without regard to any extension of time for filing, shall be deemed to be filed on such last day. If a return of withholding tax for any period ending with or within a calendar year is filed before April fifteenth of the succeeding calendar year, such return shall be deemed to be filed on April fifteenth of such succeeding calendar year.