Conn. Gen. Stat. § 12-723
The commissioner may for reasonable cause extend the time for the filing of any return, statement or other document due or required under this chapter and the payment of tax due pursuant to this chapter in accordance with regulations adopted in accordance with chapter 54. Said commissioner may require the filing of a tentative return and the payment of the tax reported to be due thereon in connection with such extension. Any additional tax which may be found to be due on the filing of a return, statement or other document as allowed by such extension shall bear interest at the rate of one per cent per month or fraction thereof from the original due date of such tax to the date of actual payment. Notwithstanding the provisions of section 12-735, no penalty shall be imposed on account of any failure to pay the amount of tax reported to be due on a return, statement or other document within the time specified under the provisions of this chapter if the excess of the amount of tax shown on the return, statement or other document over the amount of tax paid on or before the original due date of such return, statement or other document is no greater than ten per cent of the amount of tax shown on such return, statement or other document, and any balance due shown on such return, statement or other document is remitted with such return, statement or other document on or before the extended due date of such return, statement or other document.
Conn. Gen. Stat. § 12-723
( June Sp. Sess. P.A. 91-3, S. 74, 168; P.A. 95-26, S. 39, 52; P.A. 98-244, S. 30, 35; P.A. 99-121, S. 22, 28; P.A. 00-174, S. 41, 83.)