How to Amend Your CT600 After Filing: A Director's Guide

Why You Might Need to Amend Your CT600

Filing a Corporation Tax return (CT600) is rarely a one-shot process. Errors happen — perhaps you overlooked a deductible expense, misclassified a director's loan, or received a corrected set of accounts after submission. Whatever the reason, HMRC allows limited companies to amend a filed CT600 within a specific timeframe, and knowing the rules can save your company from overpaying tax or facing penalties.

The 12-Month Amendment Window

You have 12 months from the statutory filing deadline to amend your CT600. The filing deadline for a Corporation Tax return is typically 12 months after the end of your company's accounting period. So in practice, you have up to two years from the end of the accounting period itself to make corrections.

For example, if your accounting period ended on 31 March 2025, your filing deadline was 31 March 2026, and your amendment deadline is 31 March 2027. As of today, 9 August 2026, that window is still open — so act promptly if you need to make changes.

What Can You Amend?

You can amend almost any aspect of the return, including:

• Correcting income or turnover figures
• Adding expenses you forgot to include
• Adjusting capital allowances claims
• Correcting R&D tax credit claims
• Fixing errors in director's loan account entries
• Updating losses carried forward or back

Both underpayments and overpayments can be corrected. If your amendment shows you paid too much Corporation Tax, HMRC will repay the difference, usually with interest if they took more than 12 months to repay.

How to Submit an Amendment

The process depends on how you filed your original return:

• Filed online via HMRC's Corporation Tax online service: Log into your Government Gateway account, navigate to your Corporation Tax account, select the relevant accounting period, and choose to amend the return. You will resubmit the full CT600 with the corrected figures.
• Filed using third-party software (such as FreeAgent, Xero, or IRIS): Prepare the amended return within the software and resubmit it electronically. The new submission overwrites the original.
• Filed a paper return: Write to HMRC's Corporation Tax office with the corrected information. Include your company's Unique Taxpayer Reference (UTR) and clearly state which figures are changing and why.

Always keep a clear record of what changed, why it changed, and any supporting documents — such as revised invoices, updated bank statements, or corrected accounts — in case HMRC queries the amendment.

What Happens After the 12-Month Window?

If you miss the amendment deadline, you cannot simply refile. Instead, you must contact HMRC directly and explain the situation. HMRC has discretion to accept late corrections in genuine cases, particularly where there has been an overpayment of tax. You can write to the Corporation Tax office or use the HMRC Adjudicator's process if you believe HMRC has acted unfairly.

For underpayments discovered after the window closes, HMRC can raise an assessment to recover the tax owed, so it is always better to come forward voluntarily rather than wait to be discovered.

Avoid Penalties by Acting Early

Proactively amending an error — especially one that reduced your tax liability — demonstrates good faith to HMRC and can significantly reduce any potential penalties. Under the Finance Act 2007 penalty regime, unprompted disclosures attract lower penalties than those HMRC uncovers itself.

If your amendment increases the tax due, the additional amount will attract interest from the original payment deadline (usually nine months and one day after the accounting period ends). The sooner you correct the return, the less interest accrues.

Key Takeaway

Check your CT600 carefully after filing, and do not assume it is set in stone. The 12-month amendment window exists precisely because mistakes happen. If you are unsure whether a correction is needed, speak to a qualified accountant or use EasyTax's review tools to sense-check your figures before the deadline passes.