How to Amend a CT600 Tax Return: Correcting Mistakes After Filing
Filing your CT600 corporation tax return is a significant milestone, but what happens if you spot a mistake after submission? Whether it's a miscalculated figure, a missing expense claim, or an incorrect accounting period, HMRC allows amendments — but only within strict time limits.
This guide explains exactly how to correct your CT600, what deadlines apply, and how to avoid common pitfalls.
Can You Amend a CT600 After Filing?
Yes. HMRC allows companies to amend their CT600 within 12 months of the filing deadline (not the date you actually filed).
For example, if your accounting period ended 31 March 2025:
- Filing deadline: 31 March 2026 (12 months after period end)
- Amendment deadline: 31 March 2027 (12 months after filing deadline)
This gives you up to 24 months from your period end to get your return right.
What Can You Amend?
You can change virtually any part of your CT600, including:
- Turnover and income figures (boxes 145-165)
- Expense deductions and capital allowances
- Tax computation figures
- CT600 supplementary pages (trading, loans, group relief)
- iXBRL accounts and computations
What You Cannot Change
- The accounting period dates (boxes 30 and 35) — if these are wrong, you need to contact HMRC to delete the return and start fresh
- The company UTR — this is assigned by HMRC
How to Submit an Amendment
Option 1: Using Filing Software (Recommended)
The simplest way is to resubmit through CT600 filing software like Taxpipe:
- Open your filed return
- Make the necessary corrections
- Resubmit — the software marks it as an amendment automatically
HMRC's system recognises when a return is submitted for a period that already has a filed return, and treats it as an amendment.
Option 2: Using HMRC's Online Service
If you originally filed through HMRC's free service (available until 31 March 2026):
- Log into your HMRC business tax account
- Navigate to Corporation Tax
- Select the relevant accounting period
- Make your changes and resubmit
Option 3: Write to HMRC
For complex changes or if you're outside the amendment window:
Write to:
Corporation Tax Services
HM Revenue and Customs
BX9 1AX
Include your company name, UTR, the period you're amending, and clear details of the changes.
After the Amendment Deadline
If the 12-month amendment window has passed, you can still make changes by writing to HMRC, but they're under no obligation to accept them. HMRC will typically make a "discovery assessment" if they agree a change is needed.
Overpayment Relief
If you've overpaid corporation tax and the amendment deadline has passed, you may be able to claim overpayment relief under Schedule 1AB TMA 1970. This must be claimed within 4 years of the end of the accounting period.
Common Mistakes Worth Amending
1. Forgotten Capital Allowances
Many directors forget to claim Annual Investment Allowance (AIA) on equipment purchases. The AIA limit is £1,000,000 — a significant tax saving if you've bought qualifying assets.
2. Missing Expense Claims
Business expenses that reduce your taxable profit include:
- Staff costs and employer NIC
- Premises costs (rent, utilities, insurance)
- Professional fees (accountants, legal)
- Travel and subsistence
- Marketing and advertising
3. Incorrect Marginal Relief
For profits between £50,000 and £250,000, marginal relief reduces your effective tax rate. If you have associated companies, the thresholds are divided. Getting this wrong means overpaying or underpaying tax.
4. R&D Tax Credits Not Claimed
If your company does qualifying R&D work, you could claim enhanced deductions or tax credits. This is often worth thousands of pounds and frequently missed by companies filing without an accountant.
Will HMRC Investigate an Amendment?
Submitting an amendment doesn't automatically trigger an investigation. HMRC expects companies to correct genuine errors. However:
- Large changes to turnover or profit may attract attention
- Repeated amendments to the same period could raise flags
- Claims close to deadlines (especially for losses or reliefs) may be reviewed
As long as your amendment is genuine and well-documented, you have nothing to worry about.
Tax Due After Amendment
If your amendment results in more tax owed, interest runs from the original due date (usually 9 months and 1 day after your period end), not from the amendment date. Pay promptly to minimise interest charges.
If your amendment results in less tax owed, HMRC will process a refund. This typically takes 4-6 weeks.
Key Takeaways
| Aspect | Detail |
|---|---|
| Amendment window | 12 months after filing deadline |
| How to amend | Resubmit via software, HMRC online, or letter |
| Interest on underpayment | Runs from original due date |
| Overpayment relief | Up to 4 years from period end |
| Triggers investigation? | Not usually for genuine corrections |
File and Amend with Confidence
Taxpipe makes it easy to file your CT600 — and to amend it if needed. Our software handles the technical details of resubmission, ensuring HMRC processes your amendment correctly.
File your CT600 with Taxpipe → for just £59 per filing. No subscription, no hidden fees.