This guide walks through who must file, how to work through the 2026 CT-1065 instructions, which schedules and attachments you'll need, filing deadlines and extensions, and the mistakes that most often trigger a notice from Connecticut's Department of Revenue Services (DRS).
One important caveat before you start: as of this writing, DRS's current composite tax catalog still lists the 2025 edition of the CT-1065/CT-1120SI (revised 12/2025), not a confirmed 2026 annual form. Annual forms typically lag the calendar year. Always confirm the exact tax-year edition, line numbers, and rates against DRS's published release before you file.
Key Takeaways
- Connecticut business activity or Connecticut-source income can create a filing obligation, even for partnerships with minimal operations.
- CT-1065/CT-1120SI deadlines, estimated payments, and extensions follow their own calendar, not the federal Form 1065 schedule.
- Partner-level data, nonresident sourcing, and supporting schedules are mandatory parts of the return, not optional add-ons.
- CT-1065, CT-PET, and composite filings serve different purposes and often need to be coordinated together.
What Is Connecticut Form CT-1065 and What Changed for 2026?
Connecticut doesn't actually publish a standalone "CT-1065." The official document is Form CT-1065/CT-1120SI, Connecticut Composite Income Tax Return. It reports partnership income, Connecticut-source income, and partner allocations, and it's used by both partnerships and S corporations, not partnerships alone.
Your federal Form 1065 is the starting point for most figures. But Connecticut layers on its own requirements:
- State-specific additions and subtractions to federal income
- Apportionment or allocation for multistate activity
- Connecticut-source income calculations for nonresident partners
- Pass-through entity tax (PTET) elections and credits
The most recent confirmed change appears in the 2025 revision: DRS expanded the credit-summary section from four entries to seven, adding Employer CHET Contributions, the University of Connecticut Incentive Program, and Workforce Housing Opportunity Development credits. Total credits moved from Line 5 to Line 8 as a result.
Whether additional changes land in the 2026 edition isn't confirmed yet, so check the 2025 CT-1065/CT-1120SI instructions against whatever DRS releases for the current tax year.
PTET Election vs. CT-PET
Connecticut's pass-through entity tax (PTET) election is made by checking a box on a timely-filed composite return, including an extended one. It's irrevocable and can't be amended once made. The election itself doesn't replace the composite return: it adds a filing decision on top of it. The actual PTET liability and estimated payments are reported separately through Form CT-PET.
The 87.5% Credit and Nonresident Composite Filing
For 2025, partners claim a credit equal to 87.5% of the entity-level PTET paid, reported through Schedule CT-PE on individual, trust, or estate returns. Don't assume this percentage carries forward automatically into 2026. Confirm it from the current instructions before relying on it in your calculations.
Separately, composite filing requirements may obligate the partnership to report or pay tax on behalf of nonresident individual partners. This is mandatory when triggered, unlike the PTET election, which is voluntary.
How to Complete the 2026 CT-1065 Instructions
Before opening the form, gather:
- Federal Form 1065 and all schedules (Schedule K, K-1s, etc.)
- Trial balance or year-end financial statements
- Prior-year Connecticut composite return
- Ownership percentages and partner allocation agreements
- Estimated payment confirmations
- Partner tax identification numbers and residency status
Working Through the Form in Order
The return generally moves from income reporting through to balance due:
| Section | What It Covers |
|---|---|
| Entity information | FEIN, Connecticut registration, entity type, amended/final/short-period status |
| Schedule C | Federal income items (ordinary income, rentals, guaranteed payments, capital gains) in Columns A and B, with Connecticut adjustments in Column C |
| Schedule D | Connecticut-source income from subsidiary pass-through entities |
| Apportionment (Parts 2-3) | Multistate receipts calculation where separate accounting isn't available |
| Member schedules (Parts 4-6) | Partner information, Connecticut modifications, and source-income detail |
| Credits (Parts 7-8) | Member income-tax credits and PTET credit allocation |
| Schedule A | Final tax calculation, payments, balance due or refund |
Reconciling federal to Connecticut amounts is where most errors happen. Additions include non-Connecticut municipal bond interest and bonus depreciation add-backs. Subtractions include U.S. government interest and qualifying mutual fund income. Keep every separately stated item traceable back to its federal Schedule K line.
Partner-Level Reporting
Every member gets a Connecticut K-1, regardless of residency. Residents report their distributive share on Form CT-1040. Nonresidents use CT-1040NR/PY, sourcing only the Connecticut-attributable portion. Keep member numbers consistent across every schedule — Schedule B and Parts 4 through 8 all need to match, or you'll create reconciliation headaches later.
For estimated payments, verify the correct voucher, electronic-payment method, and tax period code directly from the current-year instructions. Payment categories have stayed stable in recent years, but don't assume that holds for 2026 without checking.
Don't guess. Where the composite return, CT-PET filing, and partner schedules overlap, follow the current form's line-by-line instructions rather than last year's workflow.
If your business manages accounting across multiple jurisdictions alongside Connecticut filings, coordinating the recordkeeping, payment tracking, and deadline monitoring behind this return can get complicated fast. VJM Global supports internationally managed businesses with tax-compliance coordination and accounting-record preparation, though a Connecticut-authorised tax professional should still handle or review the actual return where one is required.
What Attachments and Supporting Documents Are Required?
Attachment rules depend heavily on whether you're e-filing or paper filing.
Likely mandatory items:
- Federal Form 1065, completed before you start the state return (e-filers retain it for three years; paper filers attach it)
- Connecticut K-1s furnished to every member (not submitted with e-filed returns, but attached to authorized paper returns)
- Supplemental attachments when Schedule B exceeds eight members
Conditional, depending on circumstances:
- Apportionment documentation if using an alternative allocation method
- Subsidiary-income and rental-property statements
- Tiered-partnership member detail (names, tax IDs, ownership shares)
- Disregarded entity name/FEIN statements
- Short-period or amended-return explanations
Don't attach federal K-1s, and don't upload state K-1s with electronically filed returns. Retention isn't the same as transmission: most supporting statements just need to be kept in your records for three years, not uploaded.
Record retention checklist:
- Filed returns and all calculation worksheets
- Payment confirmations and estimated-tax vouchers
- Partner identification and allocation data
- Documentation supporting Connecticut-source income determinations
Attachment rules shift for amended, final, short-period, and superseding returns. Check the relevant subsection of the current instructions before assuming the standard list applies.
When Is CT Form 1065 Due in 2026?
Based on the most recent confirmed guidance (2025 annual return, filed in 2026), here's the applicable calendar:
| Obligation | Date |
|---|---|
| Calendar-year composite return | March 16, 2026 (next business day after March 15, which falls on a Sunday) |
| Extension deadline | September 15, 2026 |
| Fiscal-year return | 15th day of the 3rd month after year-end |
| 2026 PTET estimates (calendar year) | April 15, June 15, September 15, 2026; January 15, 2027 |
| Fiscal-year PTET estimates | 15th day of months 4, 6, 9, and the first month of the following tax year |
The 2026 PTET estimated-payment dates come directly from the 2026 CT-PET ES form. Those dates are confirmed. The annual composite deadline above reflects the 2025 fallback pattern, so confirm it has not shifted once DRS publishes the actual 2026 annual form.
Extensions: File vs. Pay
File the CT-1065/CT-1120SI EXT electronically by the original due date for an automatic six-month extension. DRS only contacts you if the extension is denied. If you already have an approved federal Form 7004, you don't need a separate reason for the state extension.
Critical point: a filing extension does not extend your payment deadline. Use Form CT-1127 separately if you need more time to pay. A federal extension also does not automatically extend your Connecticut obligations: you have to file the state extension independently.
Estimated payments are required once the annual payment obligation hits $1,000 or more. That threshold is calculated as the lesser of 90% of current-year PTET liability or 100% of the prior year's liability (assuming a full 12-month prior year was filed). Each installment should be roughly 25% of the total.
Amended, final, and short-period returns generally follow the standard due-date pattern for their respective period, but always check the specific subsection covering your situation before filing.
Who Must File CT-1065, and How Does It Compare With Form 1120-S?
Any partnership or LLC taxed as a partnership that does business in Connecticut or receives Connecticut-source income has a filing obligation, regardless of whether it turned a profit. Receiving a Connecticut K-1 from another pass-through entity also triggers the requirement.
Inactive partnerships: There's no blanket exemption for entities with no income or zero tax due. A loss year doesn't remove a filing obligation that's otherwise triggered. If the partnership is truly dormant with no Connecticut business or source income, confirm the position with DRS or a qualified preparer.
Special situations:
- Disregarded entities need a name/FEIN statement if their income flows through the return
- Tiered partnerships must document member names, IDs, and ownership shares when subsidiary payments are incomplete
- Publicly traded partnerships and entities with corporate owners may have additional reporting layers
Comparing CT-1065/CT-1120SI with Form 1120-S
Here's where things get confusing for a lot of filers: S corporations also file CT-1065/CT-1120SI, not a separate Connecticut form. The combined name reflects that both partnerships and S corps use the same state return, identifying their entity type on the form itself.
| Factor | Partnership | S Corporation |
|---|---|---|
| Federal return | Form 1065 | Form 1120-S |
| Connecticut return | CT-1065/CT-1120SI | CT-1065/CT-1120SI |
| Ownership reporting | Partner distributive shares | Shareholder pro-rata shares |
| State-source income | Allocated per partnership agreement | Allocated per ownership percentage |
| Filing deadline | Same composite deadline | Same composite deadline |
Don't pick a form based on LLC status alone. An LLC can be taxed as a partnership, an S corporation, or even a disregarded entity for federal purposes. The correct Connecticut filing follows that classification, not the "LLC" label itself. If you're unsure how your entity is classified, confirm it before selecting a form.
Common Mistakes and a Final 2026 Filing Checklist
A few errors show up repeatedly in Connecticut partnership filings:
- Using prior-year instructions or line numbers without checking for updates
- Confusing the federal Form 1065 with the state CT-1065/CT-1120SI
- Omitting partner identification numbers or residency status
- Missing Connecticut-source income calculations for nonresident partners
- Overlooking mandatory composite requirements while focusing only on the PTET election
- Assuming a federal extension automatically covers Connecticut
Mismatches between the composite return, federal 1065, CT-PET filing, and partner statements are a common source of DRS notices. Keeping member numbers and reported amounts consistent across every schedule prevents most of this.
Final pre-submission checklist:
- Confirm entity classification (partnership, S corp, disregarded entity)
- Verify the filing obligation actually applies to your situation
- Confirm the correct tax year and period (calendar, fiscal, short-period)
- Reconcile all partner ownership and allocation data
- Apply correct Connecticut modifications and apportionment
- Attach or retain required documentation per the current instructions
- Confirm signatures or electronic authorization are complete
- Verify payment confirmations and extension status
- Retain copies of everything filed, including worksheets
For multistate operations, tiered partnerships, amended returns, or anything that doesn't fit the standard pattern, review the completed return against the latest DRS instructions. Loop in a qualified tax professional before submitting.
Frequently Asked Questions
What is the extended due date for CT Form 1065 in 2026?
Based on the 2025 pattern, the extended deadline for calendar-year filers is September 15, 2026, using the CT-1065/CT-1120SI EXT form. An extension to file does not extend the time to pay.
What attachments are required with CT Form 1065?
Mandatory items include the completed federal Form 1065 and Connecticut K-1s for members. Conditional attachments cover apportionment schedules, tiered-partnership details, and short-period or amended-return explanations, depending on your situation.
Where can I find CT Form 1065 instructions and related forms (PDF) for 2026?
Check DRS's composite income tax forms library and make sure you select the current tax year before downloading anything, since annual forms can lag the calendar year.
Where can I get Connecticut tax forms, including Form 1065?
Current Connecticut tax forms come from the official DRS website. Double-check you're pulling the correct form for your entity type : CT-1065/CT-1120SI, CT-PET, composite schedules, extension, or payment vouchers all serve different purposes.
Who is required to file CT Form 1065, and do inactive partnerships need to file?
Partnerships doing business in Connecticut or earning Connecticut-source income generally must file, regardless of profitability. Inactive or no-tax-due entities should verify directly with DRS whether a return is still required in their specific circumstance.
What are the key differences between CT Form 1065 and Form 1120S for Connecticut taxpayers?
At the federal level, partnerships file Form 1065 and S corps file Form 1120-S. At the state level, both use the same combined CT-1065/CT-1120SI return, differentiated by entity classification on the form itself.
