Connecticut State Department of Revenue Services

Corporation Business Tax - FAQs

Who is required to file a Connecticut Corporation Business Tax Return?

C corporations that carry on or have the right to carry on business in Connecticut are required to file a Connecticut Corporation Business Tax Return.

Additional Information

Who Must File a Form CT-1120CU?

Groups of companies with common ownership that are engaged in a unitary business, where at least one member of the group is subject to the Corporation Business Tax, are required to file Form CT-1120CU.

If a company is individually subject to the Corporation Business Tax and does not meet the criteria to file on a combined unitary basis, it must file Form CT‑1120, Corporation Business Tax Return, and not Form CT‑1120CU.

My corporation is located out of state. Am I required to file a Corporation Business Tax Return if it is registered with the Connecticut Secretary of the State for authority to conduct business in Connecticut?

Yes. When an out-of-state C corporation obtains a certificate of authority to conduct business in Connecticut from the Secretary of the State, it is subject to the Corporation Business Tax. 

Additional Information

When is the Corporation Business Tax return due?

Every corporation must file a return on or before the fifteenth day of the month following the due date of the corporation’s corresponding federal income tax return for the income year (May 15 for calendar year taxpayers). For example, if a corporation has a December 31st year end, the return is due on May 15th. Exception for June 30th year ends: If a corporation has a June 30th year end, the return is due on October 15th.

Year End

Original Due Date

Extended Due Date

Anything except June 30

15th day of 5th month after year end

15th day of 11th month after year end

June 30

15th day of 4th month after year end

15th day of 11th month after year end

How much time will be granted to file with an appropriate extension request?

Requests that are granted will receive a six-month extension, except for those with a June 30th year end can receive a seven-month filing extension.

Additional Information

I made a Corporation Business Tax Extension payment on myconneCT before the due date. Do I still have to file the form or is the payment sufficient to get the extension approved?

Payment is not sufficient; you must timely file Form CT-1120 EXT, Application for Extension of Time to File Connecticut Corporation Business Tax Return, in order to receive an extension.

When is a corporation required to make estimated tax payments?

Every corporation subject to the Connecticut Corporation Business Tax whose estimated current year tax exceeds $1,000 must make its required annual payment in four installments.

The amount of each estimated payment is calculated based upon the corporation’s required annual payment. In general, the required annual payment is the lesser of: 

  • 90% of the tax (including surtax) shown on the return for the income year, or, if no return is filed, 90% of the tax for such year; or
  • 100% of the tax (including surtax) shown on the return for the previous income year without regard to any credit, if the previous income year was an income year of 12 months and if the company filed a return for the previous income year showing a liability for tax. 

Additional Information
Estimated Tax Payments
Estimated Tax Due Dates Required Payment Amounts
The estimated payments for the income year are the lesser of:
First ESA- Fifteenth day of the third month of the income year 30%of prior year tax (including surtax) without regard to credits or
27% of current year tax (including surtax)
Second ESB- Fifteenth day of the sixth month of the income year * 70%of prior year tax (including surtax) without regard to credits or
63% of current year tax (including surtax)
Third ESC- Fifteenth day of the ninth month of the income year
 * 80% of prior year tax (including surtax) without regard to credits or
72% of current year tax (including surtax)
Fourth ESD- Fifteenth day of the twelfth month of the income year
 * 100%of prior year tax (including surtax) without regard to credits or
90% of current year tax (including surtax)

* Taking into account all prior estimated tax payments made for this year

Which member of the Combined Unitary return should make estimated payments?

The designated taxable member is responsible for making payments on behalf of the group. Even though the designated taxable member is responsible for paying on behalf of the combined group, each taxable member is jointly and severally liable for the tax due.

How do I file and make estimated tax payments?

All Corporation Business Tax estimated payments must be filed and paid electronically. To file and pay estimated taxes electronically, visit myconneCT. 

A taxable member that wishes to act as the designated taxable member for a combined group does not have a Connecticut Tax Registration Number. How does the member obtain a Connecticut Tax Registration Number?

Every taxable member, including the designated taxable member, must have a Connecticut Tax Registration Number to file Form CT-1120CU.  If a taxable member, including the designated taxable member, does not have a Connecticut Tax Registration Number, that company must apply for one through myconneCT. In the Reason for Registering section, select Other and in the text box, type “Registering a company to file Form CT-1120CU” and follow the instructions to complete the application.  Allow ten business days for the application to be processed and the registration number to be mailed.

Please note that the designated taxable member of a combined group must be entered as a taxable member on Form CT-1120CU-MI, Combined Group Member Information. 

If the parent company of a federal consolidated return is not included in the combined group or is a non-taxable member of the combined group (i.e., it does not have nexus in Connecticut), can one of the taxable members be selected to act as the designated taxable member of the combined group?

Yes, if the parent company of a federal consolidated return is not included in the combined group or is a non-taxable member (i.e., it does not have nexus in Connecticut), then the combined group must select one of the taxable members to act as the designated taxable member for the combined group. See Special Notice 2016(1), Combined Unitary Legislation, for more information about selecting a designated taxable member and their responsibilities. 

After completing a Certificate of Dissolution with the Connecticut Secretary of the State, what forms do I need to file with the DRS?

There is no form for this specific purpose. Check the “Final return” box on the final Corporation Business Tax Return to notify the Department.

If you have questions or concerns, contact DRS at 860-297-5962, or write to us at:

Department of Revenue Services
Attn: Registration Team
450 Columbus Blvd, Suite 1
Hartford, CT 06103-1837

Are you Being Audited?

The purpose of conducting an audit is to verify that the correct tax was paid and to ensure compliance with Connecticut law. During the audit, you may be required to provide the auditor with whatever records are necessary to verify the information that was provided on the tax return.

Taxpayer Rights

You have the right to: 

  • Be informed of the audit process, the audit procedures, and appeal rights if selected for an audit
  • Have the audit conducted at a reasonable place and time, and completed in a timely manner
  • Be provided with an explanation of any changes made during an audit. 

Additional rights are described in PS 2017(1), Your Rights as a Connecticut Taxpayer.

Desk Audits and Examinations

Many examinations of returns are conducted by correspondence. Typically, an auditor will write to you to request additional information about items on your tax return, or to notify you of errors that need to be adjusted. You will have the opportunity to respond to the notice of proposed adjustments before a bill is issued.

Field Audits

An auditor will notify you by phone or correspondence that your Corporation Business Tax Return has been selected for audit. Field audits are scheduled in advance to enable enough time for you to assemble the required records.

A written confirmation of the appointment date and location, and a description of the books and records to be made available to start the audit, will be forwarded to you.

Audit Interview

At the initial meeting with you, the auditor will explain the audit approach and procedures, describe the audit process, make you aware of your appeal rights, and answer any questions that you may have about the audit. The auditor will ask you to sign the Audit Letter of Understanding and will make sure that you have a copy of Your Rights as a Connecticut Taxpayer.

Waivers/Statute of Limitations

The statute of limitations for auditing a Corporation Business Tax return is generally three years, beyond which the Department needs a written consent from the taxpayer. The auditor may ask you to sign an agreement, Consent to Extend the Statute of Limitations, which is commonly called a waiver.

Informal Conference

If you disagree with the proposed audit changes, you may request an informal conference by contacting the manager or the supervisor in the office that conducted the audit. The informal conference may be conducted by phone or in our office. You should be prepared to discuss all disputed issues and to submit the documentation necessary to support your position.

Appeal Process

If you disagree with the results of the audit, you may file a protest with our Appellate Division by using Form APL-002, Appellate Division Protest Form. The protest must be filed within 60 days of the notice of assessment.