Connecticut spreads its deadlines across three months and recently changed a rule that had been a fixture. For the 2025 tax year, individual returns are due April 15, 2026, pass-through returns March 16, 2026, and C-corporation returns May 15, 2026. The headline change: Connecticut's pass-through entity tax, mandatory through 2023, is now elective for tax years beginning in 2024 and later.

What are Connecticut's 2026 tax filing deadlines?

For the 2025 tax year, the Connecticut individual return (CT-1040) is due April 15, 2026. The pass-through entity return (CT-1065/CT-1120SI) is due March 16, 2026, since the statutory March 15 date falls on a Sunday. The C-corporation return (CT-1120) is due May 15, 2026, one month after the federal date. Three entity types, three different months.

Is Connecticut's pass-through entity tax mandatory?

Not anymore. Connecticut was the first state to enact a pass-through entity tax and required it through 2023, but for tax years beginning on or after January 1, 2024 the tax is elective. An entity opts in by checking the box on a timely-filed CT-1065/CT-1120SI; the election is annual and irrevocable for that year. The rate is 6.99%. Treat Connecticut as an elective-PTE state now, not a mandatory one.

How does Connecticut's extension work?

Individuals get an automatic six-month extension to October 15, 2026 using Form CT-1040 EXT, filed by the original due date. The extension is to file only; interest accrues at 1% per month on tax unpaid after April 15, 2026. C-corporations extend filing to November 15, 2026 with Form CT-1120 EXT.

What is Connecticut's C-corporation surcharge?

The 10% corporation business tax surcharge still applies for 2025. It is computed on Schedule C of the CT-1120 and applies to companies with at least $100 million in federal taxable income or to those filing as a combined unitary group. Smaller companies and those at the minimum tax enter zero. Confirm whether a corporate client clears the $100 million threshold before assuming the surcharge applies.

Who owes Connecticut estimated tax?

Individuals generally owe estimates when Connecticut withholding falls short of the required annual payment and the shortfall is $1,000 or more, paying quarterly on April 15, June 15, September 15, 2026, and January 15, 2027 (Form CT-1040ES). The required annual payment is the lesser of 90% of the current year or 100% of the prior year.

How does a firm handle Connecticut at scale?

The trap is the three-month spread plus the elective change. A workflow built when the PE tax was mandatory will still treat it as automatic, which is now wrong, and one keyed to a single business date will mis-file either the March pass-through or the May C-corp. Track all three dates separately, and make the PE-tax election a deliberate yearly decision rather than a default. See how we think about scaling tax prep workflows.

Where do these dates come from?

All deadlines and thresholds here trace to the Connecticut Department of Revenue Services. Verify against the source before filing.