How to Correct a CT600 Mistake: Amending Your Corporation Tax Return

How to Correct a CT600 Mistake: Amending Your Corporation Tax Return

You've just filed your CT600 and noticed a mistake. Maybe you entered the wrong turnover figure. Perhaps you forgot to claim an allowable expense. Or maybe your accountant got the tax computation wrong entirely.

Don't panic. HMRC allows you to amend your corporation tax return — but there are strict time limits, specific procedures, and potential penalties you need to understand.

This guide walks you through everything: when you can amend, how the process works, what HMRC will and won't accept, and how to avoid costly errors in the first place.

Can You Amend a CT600 After Filing?

Yes. HMRC explicitly allows companies to amend their CT600 corporation tax returns after submission. This is covered under Paragraph 15 of Schedule 18 to the Finance Act 1998.

The key rules are:

• You can amend your return within the amendment window
• You cannot amend after the window closes (you'll need to use a different process)
• Multiple amendments are allowed within the time limit
• Each amendment replaces the previous version of your return

Important: An amendment is not the same as a "discovery assessment" or an HMRC enquiry. Amendments are voluntary corrections you initiate yourself.

The CT600 Amendment Time Limit: 12 Months From the Filing Deadline

This is the single most important rule to understand:

You have 12 months from the filing deadline to amend your CT600.

Not 12 months from when you filed — 12 months from when it was due.

How to Calculate Your Amendment Deadline

The CT600 filing deadline is 12 months after the end of your accounting period. The amendment deadline is 12 months after that filing deadline — giving you effectively 24 months from the end of your accounting period.

Example:

• Accounting period ends: 31 March 2025
• CT600 filing deadline: 31 March 2026
• Amendment deadline: 31 March 2027

Example (late filer):

• Accounting period ends: 31 December 2024
• CT600 filing deadline: 31 December 2025
• You file late on 15 February 2026
• Amendment deadline: Still 31 December 2026 (12 months from the original deadline, not from when you filed)

If you filed late, your amendment window is shorter in practice. File on time to maximise your correction window.

What If You've Missed the Amendment Deadline?

If the 12-month amendment window has closed, you cannot submit a formal amendment. Instead, you have two options:

1. Write to HMRC to request an overpayment relief claim under Paragraph 51 of Schedule 18 to the Finance Act 1998 — this applies if you overpaid corporation tax
2. Make a disclosure if you underpaid tax — HMRC's voluntary disclosure process allows you to correct underpayments outside the normal amendment window

Taxpipe tip: Always file on time and review your return carefully before submission. Our CT600 calculator helps you check your figures before you file.

Common CT600 Mistakes That Require Amendment

Based on HMRC data and our experience helping thousands of directors file, these are the most frequent errors:

1. Incorrect Turnover (Box 145)

Entering gross revenue instead of net, or missing revenue from a secondary income stream. This affects your entire tax computation.

2. Missing Allowable Expenses

Forgetting to claim business expenses like office costs, professional subscriptions, travel, or staff costs. This means you overpay corporation tax.

3. Wrong Accounting Period Dates

Entering incorrect period start or end dates. This can cause HMRC's systems to flag your return as invalid.

4. Capital Allowances Errors

Claiming the wrong rate, missing eligible assets, or applying Annual Investment Allowance incorrectly. See our guide to capital allowances on your CT600 for details.

5. Incorrect Loss Figures

Misreporting carried-forward losses, failing to claim loss relief, or applying losses to the wrong period.

6. CT600 Supplementary Page Errors

Filing the wrong supplementary pages or entering data in the wrong boxes. This is especially common with CT600A (loans to participators) and CT600C (group relief).

7. Bank Interest and Investment Income

Entering bank interest in the wrong box or failing to include it at all. All interest income must be reported.

8. Directors' Loans (S455 Tax)

Failing to declare outstanding directors' loans or miscalculating the S455 tax charge.

For a comprehensive list, see our article on 12 common CT600 mistakes that cost directors money.

How to Amend Your CT600: Step-by-Step Process

Option 1: HMRC's Online Service

HMRC's free online CT600 filing service closed on 31 March 2026. If you originally filed through that service before it closed, you could amend through the same portal. For returns filed after that date, commercial software is required.

Option 2: Amend Using Commercial Software

If you filed using commercial software, you can typically amend through the same platform:

1. Log in to your account
2. Open the filed return
3. Make corrections to the relevant figures
4. The software generates an amended CT600 and updated iXBRL computation
5. Resubmit electronically to HMRC

This is the recommended approach. Commercial software validates your figures, ensures iXBRL compliance, and keeps an audit trail of changes. Check with your software provider that they support amendment filing for your specific return.

Option 3: Paper Amendment

You can also submit a paper amendment by post, though this is slower and not recommended:

• Download form CT600 from GOV.UK
• Complete the full return (not just the changed boxes)
• Write "AMENDMENT" clearly on the front
• Post to: Corporation Tax Services, HM Revenue and Customs, BX9 1AX

Paper amendments typically take 6–8 weeks to process.

What Happens After You Amend

Once HMRC receives your amended return:

• If you owe more tax: HMRC will issue a revised tax calculation showing the additional amount due, plus any interest from the original payment deadline
• If you've overpaid: HMRC will process a corporation tax refund, usually within 4–6 weeks
• If HMRC disagrees: They may open an enquiry into the amendment (rare for straightforward corrections)

Penalties for CT600 Errors

HMRC applies penalties based on the behaviour that caused the error, not just the error itself. The penalty regime is set out in Schedule 24 of the Finance Act 2007.

Penalty Ranges by Behaviour Type

Key Points About Penalties

• Genuine mistakes where you took "reasonable care" attract no penalty — HMRC recognises that honest errors happen
• Self-correcting (amending your own return voluntarily) is treated as an "unprompted disclosure" and attracts the lowest penalties
• HMRC-discovered errors attract higher penalties because they're "prompted"
• You can appeal penalties if you believe you took reasonable care

How to Reduce or Avoid Penalties

1. Amend promptly — the sooner you correct an error, the more favourably HMRC treats it
2. Keep records — show HMRC you took reasonable care by maintaining good accounting records
3. Use reliable software — HMRC considers whether you used appropriate tools and took advice
4. Cooperate fully — if HMRC contacts you about an error, respond promptly and transparently
5. Disclose voluntarily — unprompted disclosures always attract lower penalties than HMRC-discovered errors

Did you know? Filing with Taxpipe creates a full audit trail of your tax computation, giving you evidence of reasonable care if HMRC ever queries your return.

Interest on Underpayments and Overpayments

Regardless of penalties, HMRC charges interest on late-paid corporation tax:

• Late payment interest rate: Currently 7.75% per annum (from 9 January 2026 — check HMRC's current rates)
• Interest runs from: The original payment deadline (9 months and 1 day after your accounting period end)
• Interest is charged even if: You amend voluntarily — the tax was still paid late

Conversely, if your amendment shows you overpaid:

• HMRC repayment interest rate: Currently 2.75% per annum
• Repayment interest runs from: The date you made the overpayment

HMRC Enquiries vs. Amendments

It's important to understand the difference:

If you discover an error, it's always better to amend yourself than to wait for HMRC to find it. Self-correction demonstrates good faith and triggers lower penalties.

How Taxpipe Helps You Avoid CT600 Mistakes

The best amendment is the one you never need to make. Here's how Taxpipe helps you get it right first time:

Built-In Validation

Taxpipe validates your CT600 before submission, checking for:

• Mathematical errors in your tax computation
• Missing mandatory boxes
• Inconsistencies between figures
• Common data entry mistakes

Guided Filing Flow

Our step-by-step interface walks you through each section of the CT600, with plain-English explanations of what each box means. No accounting jargon, no guesswork.

Automatic iXBRL Generation

Taxpipe automatically generates HMRC-compliant iXBRL tax computation documents for every filing. For micro-entity (FRS 105) and dormant company accounts, iXBRL is generated automatically too — no separate tagging software needed for those filing types.

Audit Trail

Every calculation and entry is logged, giving you a clear record of how your tax figure was computed — essential evidence of "reasonable care" if HMRC ever queries your return.

Ready to file with confidence? Sign up for Taxpipe — accurate CT600 filing for just £59 per return.

What If You Discover a Mistake Years Later?

If you find an error outside the 12-month amendment window:

You Overpaid Corporation Tax

• Claim overpayment relief under Paragraph 51 of Schedule 18 to the Finance Act 1998
• Time limit: 4 years from the end of the accounting period
• Write to HMRC with details of the error and the overpaid amount
• Include supporting evidence (corrected computations, bank statements, etc.)

You Underpaid Corporation Tax

• Make a voluntary disclosure through HMRC's disclosure facility
• Pay the outstanding tax plus interest
• Penalties will be at the lower "unprompted" rate because you came forward voluntarily
• HMRC can go back up to 4 years for careless errors, 6 years for deliberate errors, and 20 years for deliberate concealment

Frequently Asked Questions

How long do I have to amend my CT600?

You have 12 months from the filing deadline to amend your CT600. The filing deadline is 12 months after the end of your accounting period, so you effectively have 24 months from the end of your accounting period to submit an amendment.

Is there a penalty for amending my CT600?

Not automatically. If your amendment corrects a genuine mistake where you took reasonable care, there is typically no penalty. Penalties apply based on the nature of the error — careless errors attract 0%–30%, while deliberate errors can attract up to 100% of the additional tax due.

Can I amend my CT600 more than once?

Yes. You can submit multiple amendments within the 12-month amendment window. Each amendment replaces the previous version of your return.

What if I amend and then owe more tax?

HMRC will issue a revised calculation showing the additional tax due, plus interest from the original payment deadline (9 months and 1 day after your accounting period end). You should pay promptly to minimise interest charges.

Can HMRC reject my amendment?

HMRC generally processes amendments automatically. However, they can open an enquiry into your amended return if they have concerns. They have 12 months from the date of your amendment to open an enquiry.

How do I amend a CT600 now that HMRC's free filing service has closed?

HMRC's free CT600 filing service closed on 31 March 2026. To file or amend a return now, you need HMRC-recognised commercial software. Sign up for Taxpipe to file your next return accurately from the start.

What records should I keep to support an amendment?

Keep your original return, the amended return, all supporting computations, bank statements, invoices, and a written explanation of what changed and why. Taxpipe maintains a full audit trail automatically.

Can my accountant amend my CT600 for me?

Yes — if you have an accountant, they can submit an amendment on your behalf using their professional software. If you're a director handling your own filings, Taxpipe makes it easy to get your return right from the start with our guided filing flow.

Key Takeaways

• You can amend your CT600 within 12 months of the filing deadline
• Self-correcting is always better than waiting for HMRC to discover errors
• Genuine mistakes with reasonable care attract no penalty
• Act quickly — interest accrues from the original payment deadline
• Use reliable filing software like Taxpipe to minimise errors and maintain an audit trail
• Keep detailed records to demonstrate reasonable care

File your CT600 with confidence. Try Taxpipe today — just £59 per return, with built-in validation, guided filing, and a complete audit trail. Calculate your corporation tax now →

This guide is for informational purposes only and does not constitute tax advice. If you have a complex amendment or are facing an HMRC enquiry, consider consulting a qualified tax adviser. Taxpipe is a CT600 filing tool — we help you file accurately, but we don't provide advisory services.

Last updated: June 2026

Related Reading

• Corporation Tax Interest: Late Payment Rates, Repayment Interest & How to Avoid Charges
• Corporation Tax Penalties & Interest Charges
• What Happens If You Miss the CT600 Deadline? Penalties, Interest & How to Fix It