Introduction

Submitting a Corporation Tax Return is an important responsibility for many UK limited companies. Understanding how to file a company tax return helps directors report their company's taxable profits accurately, meet HM Revenue and Customs (HMRC) requirements, and reduce the risk of avoidable filing penalties.

The CT600 is the form companies generally use to declare their Corporation Tax position to HMRC. It is normally submitted alongside the company's accounts and tax computations, which show how its taxable profits and tax liability have been calculated.

This guide explains the key stages of completing and submitting a CT600 in 2026, from preparing company accounts and calculating taxable profits to checking tax reliefs and meeting filing and payment deadlines.

What Is a CT600 Corporation Tax Return?

A CT600 is the main form used by companies to report information relevant to their Corporation Tax liability. It sets out the company's financial and tax details for the relevant accounting period.

Depending on the business, the return may include:

• The start and end dates of the accounting period.

• Trading income and other taxable income.

• Taxable profits or losses.

• Allowable deductions and tax adjustments.

• Capital allowances and eligible reliefs.

• The Corporation Tax calculation.

• Details of relevant tax payments and other information.

Companies generally submit the CT600 with supporting accounts and tax computations.

Although company accounts prepared for Companies House provide the starting point for tax reporting, accounting profit and taxable profit are not always the same. Certain expenses may need to be added back, while eligible allowances and reliefs may reduce the amount subject to tax.

Which Companies Must Submit a Corporation Tax Return?

Many UK limited companies must file a Corporation Tax Return when HMRC requires one. This obligation can apply even if the company has made a loss or does not owe Corporation Tax for the accounting period.

A company that has stopped trading may be considered dormant for Corporation Tax purposes, depending on its circumstances. However, directors should confirm the company's status with HMRC rather than assume that no return is necessary.

If HMRC issues a notice requiring a Corporation Tax Return, the company should respond to that notice unless HMRC formally withdraws the requirement.

Checking the company's filing obligations early gives directors time to prepare the necessary records and avoid last-minute compliance issues.

Step 1: Confirm the Accounting Period

Before preparing the CT600, establish the exact accounting period that the return must cover.

The Corporation Tax accounting period determines the period for which taxable profits are calculated. It may differ from the company's financial year or the period covered by its Companies House accounts.

For example, a newly incorporated company may have an initial accounting period that does not match its usual financial reporting cycle. If an accounting period exceeds 12 months, the company may need to submit two Corporation Tax Returns.

Check the start and end dates carefully before entering any figures. Incorrect dates can lead to calculation errors, incomplete reporting, or the submission of a return for the wrong period.

Step 2: Prepare the Company Accounts

After confirming the accounting period, gather and prepare the company's financial records. These accounts form the foundation for calculating taxable profits and completing the Corporation Tax Return.

Depending on the company's activities, the records may include:

• Sales revenue and other business income.

• Cost of sales and operating expenses.

• Business assets and liabilities.

• Outstanding customer invoices.

• Unpaid supplier bills.

• Bank balances and cash transactions.

• Director loans and related transactions.

• Payroll costs and employment expenses.

Reconcile bank statements, review unpaid invoices, and ensure that income and expenses have been recorded in the correct accounting period.

Complete and reliable accounts make it easier to identify discrepancies before preparing the Corporation Tax calculation. Missing information should be investigated rather than estimated without appropriate support.

Step 3: Calculate the Company's Taxable Profit

The profit shown in a company's accounts is not necessarily the figure used to calculate Corporation Tax. Tax rules determine which expenses, deductions, and allowances can be applied when arriving at taxable profits.

The calculation may require adjustments such as:

• Adding back expenses that are not deductible for Corporation Tax.

• Claiming eligible capital allowances.

• Applying qualifying losses from earlier periods where permitted.

• Reviewing relevant expenditure and available tax reliefs.

• Making other adjustments required by the company's circumstances.

After the relevant adjustments have been made, the resulting taxable profit can be used to calculate the company's Corporation Tax liability.

Keep supporting documents and clear calculations for each adjustment. These records help demonstrate how the figures were reached if HMRC requests further information.

Step 4: Review Available Tax Reliefs

Before completing the return, check whether the company may qualify for any tax reliefs, allowances, or deductions.

Depending on its activities and expenditure, a business may qualify for relief relating to capital investment, research and development, or other eligible costs.

Companies undertaking qualifying innovation activities should review the R&D tax relief eligibility rules to understand the relevant conditions and determine whether a claim may be appropriate.

Eligibility should always be assessed against the applicable requirements. A company should not claim a relief simply because it has incurred costs that appear relevant. Appropriate evidence must be retained to support any claim made.

Reviewing potential reliefs before finalising the CT600 can help ensure that the return reflects the company's tax position accurately.

Step 5: Review the Corporation Tax Computation

Once taxable profits have been calculated and potential reliefs considered, carefully check the Corporation Tax computation before transferring the final figures to the CT600.

Key checks include:

• Confirming that all relevant income has been included.

• Identifying expenses that are not deductible for tax purposes.

• Reviewing capital allowance calculations.

• Checking the treatment of losses.

• Confirming that eligible reliefs have been applied correctly.

• Verifying the applicable Corporation Tax rates.

• Checking the final liability and any relevant payments.

Investigate any unexpected figures or differences between the accounts and tax computations before submitting the return.

A thorough review can help identify mistakes early, reducing the likelihood of having to amend the return later.

Step 6: Complete the CT600 Form

Once the accounts and tax computations are ready, use HMRC-compatible filing software to complete the CT600.

The form asks for information about the company, its accounting period, taxable profits, relevant deductions, and Corporation Tax liability.

Some businesses may also need supplementary pages, depending on their income sources, claims, or other circumstances. These additional sections provide information that cannot be covered fully in the main return.

Enter only details that accurately reflect the company's circumstances. Check that the figures match the supporting accounts and tax computations before proceeding.

If the company has complex transactions or an unusual tax position, professional advice may help ensure that the return is completed correctly.

Step 7: Submit the Corporation Tax Return Online

Most companies must submit their Corporation Tax Returns electronically using compatible software.

Before filing, review the completed CT600 and its supporting information to make sure all required details are present and consistent. Correct any errors identified during the final checks.

Once the return has been submitted, save the filing confirmation and retain a copy of the submitted form, accounts, and tax computations. These records may be useful if HMRC asks questions or the company needs to review its previous tax position.

Do not leave submission until the last moment. Preparing the return in advance allows time to resolve missing information, discrepancies, or technical issues before the deadline.

Step 8: Pay Corporation Tax on Time

Submitting the CT600 and paying the Corporation Tax owed are separate obligations. Filing the return does not automatically pay the tax due.

For many companies, Corporation Tax is payable nine months and one day after the end of the relevant accounting period. Companies meeting the applicable conditions for large-company status may have to pay through quarterly instalments.

Directors should estimate the company's expected tax liability well in advance and plan how the payment will be funded. Maintaining a cash-flow forecast and setting money aside for tax can help prevent unexpected pressure on working capital.

Always confirm the payment deadline and payment arrangements that apply to the specific company, as not every business follows the same schedule.

How Much Does It Cost to File a CT600?

The cost of preparing and submitting a Corporation Tax Return depends on the company's size, the condition of its financial records, and the complexity of its tax affairs.

A business with organised bookkeeping and straightforward transactions may require less preparation work than a company with multiple income sources, complicated transactions, or significant tax adjustments.

Factors that can affect the fee include:

• Annual turnover and the number of transactions.

• The quality of existing bookkeeping records.

• Employee numbers and payroll requirements.

• VAT registration and related accounting work.

• Company assets and capital expenditure.

• Tax relief claims and loss calculations.

• The overall complexity of the company's finances.

Reviewing Corporation Tax return costs can help directors understand what influences professional fees and compare the cost of handling the work internally with appointing an accountant.

When requesting a quotation, confirm whether it includes annual accounts, the CT600, tax computations, and any additional advice required.

Keep Payroll Records Accurate

Companies employing staff must also meet their payroll and PAYE responsibilities.

Employee salaries, employer National Insurance contributions, and other employment costs can affect the company's accounts and Corporation Tax calculation. Missing or inaccurate payroll information may therefore create additional work during year-end reporting.

Maintaining reliable payroll records for your business helps ensure that employment expenses are recorded accurately and that the necessary information is available when preparing the annual accounts.

Regularly reconciling payroll figures can also make the year-end process smoother and help identify discrepancies before the return is submitted.

Common Mistakes to Avoid When Filing a CT600

Even companies with relatively straightforward finances can make errors if the information is not checked carefully.

Using the Wrong Accounting Period

Verify the dates covered by the return before entering financial information. Incorrect dates may result in figures being reported for the wrong period.

Treating Accounting Profit as Taxable Profit

Accounting profit may require adjustments under Corporation Tax rules. Calculating tax directly from the accounting profit without reviewing those adjustments can produce an incorrect liability.

Missing Eligible Tax Reliefs

Failing to consider available allowances and reliefs may cause a company to overlook legitimate opportunities to reduce its Corporation Tax bill.

Claiming Expenses That Do Not Qualify

Not every expense recorded in the accounts is deductible for Corporation Tax purposes. Review the tax treatment of relevant costs before including them in the calculation.

Failing to Reconcile Records

The figures in the CT600 should agree with the supporting accounts and tax computations. Investigate discrepancies before filing rather than leaving them unexplained.

Overlooking the Payment Deadline

Remember that submitting the return and paying the tax are separate responsibilities. Record both deadlines and make arrangements to meet them.

What Happens If a CT600 Is Filed Late?

HMRC may impose penalties when a Corporation Tax Return is submitted after the applicable deadline. Interest and additional penalties may also arise if the tax itself is paid late.

The potential consequences can become more serious the longer a required return remains outstanding. Late filing may also create unnecessary administrative work for the company.

Prepare the accounts and tax calculations well before the deadline so there is sufficient time to review the information and correct any problems.

If the deadline has already passed, act promptly. Review any correspondence from HMRC, establish which obligations remain outstanding, and take steps to resolve the issue.

How to Make Corporation Tax Filing Easier

Good financial management throughout the year can reduce the effort required to prepare and submit the CT600.

Businesses can improve their process by following these practices:

1. Update accounting records regularly.

2. Reconcile business bank accounts and investigate differences.

3. Keep sales invoices, purchase invoices, and receipts organised.

4. Maintain complete and accurate payroll records.

5. Record business expenses and capital purchases appropriately.

6. Review eligibility for relevant tax reliefs.

7. Add Corporation Tax filing and payment dates to a calendar.

8. Retain copies of previous returns and tax computations.

9. Estimate the expected tax liability before payment is due.

10. Seek professional advice when transactions or tax requirements are complicated.

Following these steps helps companies maintain accurate records, reduce last-minute pressure, and manage their tax responsibilities more effectively.

Should You Hire an Accountant to Complete Your CT600?

Some companies with simple financial affairs may have the knowledge and resources to prepare their Corporation Tax Return internally. However, professional accounting support can be particularly useful when the business has complex accounts, multiple income sources, significant capital expenditure, or uncertainty about tax reliefs.

An accountant can help prepare the company's accounts, calculate taxable profits, review deductions, complete the CT600, and manage the filing process.

Professional support can also help directors understand their anticipated tax liability and plan for the payment deadline. The appropriate level of assistance will depend on the company's financial circumstances, the expertise available internally, and the complexity of its tax affairs.

Final Thoughts

Knowing how to file a company tax return is an important part of running a UK limited company. The process involves confirming the correct accounting period, preparing reliable accounts, calculating taxable profits, checking available reliefs, and submitting the CT600 to HMRC.

Directors should remember that filing the return is only one part of their Corporation Tax responsibilities. The tax must also be paid by the relevant deadline, and supporting records should be retained in case they are needed later.

For businesses that need assistance with Corporation Tax, bookkeeping, payroll, and other accounting requirements, MyIVA offers professional accounting support to help UK companies keep their financial records organised and manage their tax responsibilities more efficiently.