How Do I Amend a CT600?

If you've spotted an error on your company tax return after filing it with HMRC, you can usually correct it by submitting an amended CT600. Most amendments are straightforward — you have up to 12 months from the original filing deadline to submit a corrected return online.

This guide explains how the amendment process works, what you can and cannot change, and what happens to any tax you've already paid.

When Can You Amend a CT600?

HMRC allows you to amend a company tax return within 12 months of the statutory filing deadline. The filing deadline itself is 12 months after the end of your accounting period — so in practice, you have up to 24 months from the end of your accounting period to file a correction.

Example: If your accounting period ends on 31 March 2024:

  • Filing deadline: 31 March 2025
  • Last date to amend: 31 March 2026
Once this 12-month amendment window closes, you can no longer submit a correction online. You may still be able to ask HMRC to make changes by writing to them, but this is at their discretion and generally applies where additional tax is owed.

What Can You Amend?

You can use an amended CT600 to correct most types of error or change. Common reasons to amend include:

  • Missing deductions — you forgot to claim an allowable business expense
  • Capital allowances — you under-claimed or over-claimed allowances for equipment, vehicles, or property
  • R&D tax credits — you decided to claim Research and Development relief after the original submission
  • Revised accounts — your statutory accounts were revised after the return was filed, changing the profit or loss figure
  • Losses — you need to carry back a loss from a later period to offset against profits in the return being amended
  • Incorrect period dates — the accounting period start or end dates were wrong
  • Rounding or calculation errors — minor discrepancies that affected the final tax figure
  • Omitted income — you realise income was missed from the original return, in which case additional tax will be due
You cannot use an amendment to change your accounting period itself. If the fundamental period dates are wrong, contact HMRC to discuss your options.

How to Amend a CT600 Online

The simplest way to amend a CT600 is by resubmitting through HMRC's Corporation Tax Online service or through CT600-compliant software. The process is broadly the same as the original submission — you enter the corrected figures and file the return again for the same accounting period.

Step-by-step:

  1. Log in to HMRC Online Services or open your CT600 software
  2. Navigate to the correct company and accounting period
  3. Update the boxes that need correcting — for example, if you missed an allowable expense, adjust the expenditure figure and let the software recalculate the tax
  4. Review the amended tax calculation and confirm the figures are correct
  5. Submit the amended return to HMRC
HMRC will compare the amended return to the original and adjust your account accordingly. You do not need to contact HMRC separately to notify them — the online submission is sufficient.

If you need to correct multiple boxes, change them all in one submission rather than filing separate amendments. Each amendment resets the enquiry window for the matters covered.

What Happens After You Amend?

If your amendment reduces the tax owed:

HMRC will credit your account with the overpayment. If you have already paid the original amount, you can request a refund or offset it against future corporation tax. In some circumstances HMRC also pays repayment interest on amounts overpaid.

If your amendment increases the tax owed:

You will need to pay the additional amount. Corporation tax is normally due 9 months and 1 day after the end of your accounting period — if you are filing an amendment after that date, the additional tax will already be overdue. HMRC charges interest on late payments from the original due date, so pay promptly once you have submitted the amendment.

Amending Outside the 12-Month Window

If more than 12 months have passed since the filing deadline, you can no longer amend online. At that point you will need to write to HMRC:

HM Revenue and Customs Corporation Tax Services BX9 1AX

Your letter should include:

  • The company's full name and Unique Taxpayer Reference (UTR)
  • The accounting period the amendment relates to
  • A clear explanation of what changed and why
  • The corrected figures, ideally with supporting calculations
HMRC will review the request. If you are disclosing additional income or tax owed, HMRC generally takes a cooperative approach. If you are claiming a repayment outside the normal amendment window, approval is less certain.

Amended Returns and HMRC Enquiries

Submitting an amendment does not automatically trigger an HMRC compliance check, but it does reset the enquiry window for the matters covered. HMRC normally has 12 months from the filing date to open an enquiry — when you amend, that window reopens from the amendment date for the specific areas the amendment touches.

If HMRC discovers errors during a compliance check that you did not correct yourself, penalties may apply. Errors classed as careless carry a penalty of up to 30% of the tax owed; deliberate errors can attract higher rates. Voluntarily correcting errors through an amendment reduces or eliminates penalties in most cases.

For guidance on errors that commonly attract HMRC attention, see common mistakes on company tax returns.

Can I Amend After HMRC Has Opened an Enquiry?

Once HMRC has opened a formal enquiry into your return, the normal amendment process is suspended for the matters under enquiry. You cannot amend those specific areas until the enquiry is concluded. You can still amend unrelated parts of the return if they fall within the 12-month amendment window.

If you identify an error while an enquiry is in progress, raise it as part of your enquiry correspondence rather than attempting to amend independently.

Do You Need Software to Amend?

HMRC does not provide its own CT600 filing tool — companies must use approved third-party software. If you filed your original return using CT600 software, you can use the same package to prepare and submit the amendment. If you no longer have access to the original software, you will need an alternative package that can file for the same accounting period.

Summary

Amending a CT600 is usually straightforward if you act within 12 months of the filing deadline. Open the return in your CT600 software, correct the figures, and resubmit — no separate notification to HMRC is needed. If the amendment increases tax owed, pay the additional amount promptly to limit interest. Outside the 12-month window, write to HMRC directly. For a reminder of the key dates that apply to your company, see company tax return deadlines.