Scope: this guide covers the 2025 tax year return filed in 2026. If your entity files on a fiscal year, or you're planning for a 2026 tax year return filed in 2027, confirm dates with the Connecticut Department of Revenue Services (DRS) before relying on anything here.
We'll cover who files, the 2026 dates, which form to use, how to file and pay, and what to do if you've already missed the deadline.
Key Takeaways
- The Connecticut extension form is CT-1065/CT-1120SI EXT, and it must be filed electronically.
- For calendar-year 2025 returns, the extended due date is September 15, 2026.
- An extension to file does not extend the time to pay.
- Federal Form 7004 does not automatically extend the Connecticut deadline.
What Is the CT-1065 Extension and Who Uses It?
What CT-1065/CT-1120SI actually is
DRS calls Form CT-1065/CT-1120SI the Connecticut Composite Income Tax Return. It's not federal Form 1065, which is the partnership return filed with the IRS. The Connecticut form is a state return that covers both partnerships (CT-1065) and S corporations (CT-1120SI).
It is also separate from Form CT-PET, the Connecticut Pass-Through Entity Tax. CT-PET is optional, elected annually, and DRS states the election can't be amended or revoked.
Who has to file
DRS's 2025 instructions cover:
- General partnerships and limited partnerships
- Limited liability partnerships
- S corporations
- LLCs treated as partnerships
According to DRS composite tax guidance, a pass-through entity doing business in Connecticut, or earning income from Connecticut sources, must file regardless of income or loss. An entity that receives a Schedule CT K-1 from another pass-through entity must file too.
The "LLC" label alone doesn't pick your form. Federal tax classification and Connecticut activity do.
What the extension does and doesn't do
Form CT-1065/CT-1120SI EXT gives a six-month extension to file the return and furnish Schedule CT K-1 to members. It does not erase interest, penalties or a balance due. It also doesn't cover CT-PET, which has its own extension form, CT-PET EXT.
CT-1065 Extension Due Dates for 2026
Keep tax year and filing year straight
A calendar-year 2025 return is filed in 2026. A calendar-year 2026 return is filed in 2027. Label every date with both years, or you'll apply the wrong deadline.
Calendar-year 2025 deadlines
- Original due date: March 15, 2026 falls on a Sunday, so the next-business-day rule moves timely filing to March 16, 2026.
- Extension request deadline: the same date as the original return, March 16, 2026.
- Extended due date: September 15, 2026.
DRS treats a filing on the next business day as timely whenever a due date lands on a weekend or legal holiday.
Fiscal-year and short-period filers
Don't reuse the calendar-year dates. The original due date is the 15th day of the third month after your taxable year closes. File the extension by that date. The extension runs six months, subject to the same next-business-day rule.
For short-period, final or amended returns, check the current DRS instructions against your actual year-end.
Federal Form 7004 versus the Connecticut extension
A federal extension does not automatically extend the Connecticut filing date. You still file a separate CT-1065/CT-1120SI EXT by the Connecticut due date.
If you filed Form 7004 federally, DRS says you don't need to give a reason on the Connecticut request. If you didn't, you may request the extension for reasonable cause.
CT-PET dates
CT-PET follows the same 15th-day-of-month-three schedule, so for calendar-year 2025 it's due March 16, 2026. Its own CT-PET EXT requests an extension to September 15, 2026. The estimated CT-PET payment coupon is a separate item, listed as January 15, 2026 on the DRS 2026 calendar.
Quick-reference timeline (calendar-year 2025 return)
| Item | Date |
|---|---|
| CT-PET estimated payment | January 15, 2026 |
| Original return due; extension request and payment due | March 16, 2026 |
| Extended return due (CT-1065/CT-1120SI) | September 15, 2026 |
| Extended CT-PET return due (with CT-PET EXT) | September 15, 2026 |
Dates can change. Check the current DRS tax calendar before you file.
How to File the CT-1065 Extension and Handle Payment
Step 1: Confirm the entity and filing requirement
Before touching the form, confirm:
- Federal tax classification (partnership, S corporation or other)
- Connecticut business activity or Connecticut-source income
- The tax year
- Prior-year filings and estimated payments made
Reconcile the federal return information, Connecticut-source income and member data now. It saves rework when the full return is prepared.
Step 2: Use the correct form
For the composite return, use Form CT-1065/CT-1120SI EXT (2025 edition, Rev. 12/25). If you've elected CT-PET, you'll also need CT-PET EXT. Always pull the current edition from DRS, because older forms get rejected.
Step 3: File electronically
Per the 2025 EXT form instructions, the extension must be filed and paid electronically. DRS lists two channels:
- myconneCT, the DRS online portal
- The Modernized e-File (MeF) program, typically used through a tax professional's software
Paper isn't an option for this form. Save the confirmation.
Step 4: Estimate the tax and pay by the original deadline
On Line 1, enter your estimated Connecticut composite income tax liability for 2025. An estimate is allowed, but an amount is required. If no tax is due, enter zero. To get the extension, file by the return due date and pay the Line 1 amount.
Keep the taxes straight:
- Line 1 on the composite extension covers composite income tax only.
- CT-PET is a separate optional tax with its own return, extension and payment reporting.
- Form CT-1127 is the form DRS names for requesting additional time to pay. Don't assume it's granted; confirm current rules first.
Step 5: Keep proof
Hold onto these in one folder:
- The submitted extension and filing confirmation
- Payment confirmation
- Your tax computation
- Federal return information
- Partner or shareholder data
- Any DRS correspondence
If a notice arrives about a mismatched payment or a disputed timely filing, this folder settles it quickly.
Documents, Common Mistakes, and Post-Deadline Issues
Gather before you prepare the return
- Federal Form 1065 or 1120-S information
- Trial balance or financial statements
- Prior-year Connecticut return
- Ownership and residency data for every member
- Connecticut-source income calculations
- Estimated-payment records and applicable schedules
Mistakes that cost money
- Treating the federal extension as the Connecticut extension
- Choosing a form based on the LLC label alone
- Using an outdated edition of the DRS form
- Skipping CT-PET EXT when a CT-PET filing applies
- Leaving Line 1 blank instead of entering zero
- Assuming the extension delays payment
- Selecting the wrong tax year, or reporting inconsistent member data
- Not saving filing confirmations
Penalties and interest
The 2025 EXT instructions state a 10% penalty on tax unpaid by the original due date, plus interest of 1% per month or fraction of a month until paid.
There is a narrow relief. When a filing extension is granted, no late-payment penalty applies if the unpaid balance is no more than 10% of the tax shown due and you pay it by the extended due date.
If no tax is due and a required return is filed late, DRS may impose a $50 late-filing penalty.
CT-PET carries the same 10% and 1% rules. If DRS files a missing CT-PET return for you, the penalty is 10% of the balance due or $50, whichever is greater.
If you missed the deadline
Don't wait. File the underlying return and pay as soon as you can, because interest accrues every month. Then contact DRS or a Connecticut-authorised tax professional about penalty options. Reasonable-cause relief exists, but you have to ask.
Final 2026 CT-1065 Extension Checklist and Next Steps
Work through this in order:
- Confirm tax year and entity classification.
- Verify the DRS deadline (calendar-year 2025: March 16 request, September 15 extended).
- Select CT-1065/CT-1120SI EXT.
- Decide whether CT-PET EXT is also needed.
- File through myconneCT or MeF.
- Pay the Line 1 estimate by the original due date.
- Save every confirmation.
Forms and deadlines can change after publication, so check the current Connecticut DRS forms library and instructions before you file.
VJM Global supports US businesses with accounting-record preparation and broader tax-compliance coordination, including Form 1065 and other federal and state filings prepared under IRS protocols. State-specific Connecticut returns should still be reviewed or filed by a Connecticut-authorised tax professional where required.
Frequently Asked Questions
What is the extended due date for CT Form 1065?
For a calendar-year 2025 return filed in 2026, the extended due date is September 15, 2026. You request it on Form CT-1065/CT-1120SI EXT by March 16, 2026.
What are CT-1065 and CT-1120SI?
They are Connecticut's combined composite income tax return for partnerships (CT-1065) and S corporations (CT-1120SI). They are separate from federal Forms 1065 and 1120-S.
Can I file my CT-1065 online?
For the extension, online filing is required. DRS accepts it through myconneCT or the Modernized e-File program, and payment must also be made electronically.
Does a CT-1065 extension extend the time to pay?
No. The extension covers filing only. Tax is still due by the original date, and unpaid amounts accrue a 10% penalty and 1% monthly interest. Form CT-1127 is the form DRS names for payment-extension requests.
Do I need a separate Connecticut extension if I filed federal Form 7004?
Yes. Federal Form 7004 doesn't automatically extend the Connecticut date. File CT-1065/CT-1120SI EXT separately by the Connecticut due date.
