# How do I file a CT600 Corporation Tax return?

Last updated: 19 October 2026
Category: Corporation Tax

**Quick answer:** File your CT600 online through HMRC's Company Tax Return service or commercial software. You need your company UTR, accounts, and tax computations. The deadline is 12 months after your accounting period ends.

## Detailed explanation

**Filing your CT600 Corporation Tax return**

The CT600 is the form your company uses to report its taxable profits and calculate the Corporation Tax due to HMRC.

**What you need before filing**

1. **Company Unique Taxpayer Reference (UTR)** - 10-digit number from HMRC
2. **HMRC online account** - registered for Corporation Tax
3. **Annual accounts** - profit and loss account, balance sheet
4. **Tax computations** - adjustments from accounting profit to taxable profit
5. **Supporting documents** - capital allowance calculations, loss relief claims

**Key deadlines**

| Action | Deadline |
|---|---|
| Pay Corporation Tax | 9 months + 1 day after period end |
| File CT600 | 12 months after period end |
| Amend a return | 12 months after the filing deadline |

**How to file**

**Option 1: Commercial software (recommended)**
- Software like AccountsOS, Xero, FreeAgent, or TaxCalc
- Prepares accounts in iXBRL format automatically
- Files directly to HMRC
- Handles tax computations

**Option 2: HMRC Company Tax Return service**
- Free but limited
- Suitable for simple companies
- Must prepare iXBRL accounts separately
- Access via Government Gateway

**What the CT600 includes**

- **Box 1-9**: Company details, accounting period, UTR
- **Box 145**: Turnover
- **Box 155**: Trading profits
- **Box 160**: Net trading profits after capital allowances
- **Box 235**: Total profits chargeable to Corporation Tax
- **Box 440**: Tax due
- **Box 470**: Tax payable (after deductions)

**Common adjustments from accounts profit to taxable profit**

You can't just use your accounts profit figure. Common adjustments include:

| Addition (add back) | Deduction |
|---|---|
| Depreciation | Capital allowances |
| Client entertaining | R&D enhanced expenditure |
| Fines and penalties | Patent Box relief |
| Non-business expenses | Brought-forward losses |
| Provisions | Creative industry relief |

**iXBRL accounts**

HMRC requires accounts in **inline XBRL** format. Most commercial software generates this automatically. If using free tools, you'll need HMRC's free tagging service or a third-party converter.

**Filing for the first time?**

1. Register for Corporation Tax within 3 months of trading
2. Receive your UTR (can take several weeks)
3. Register for HMRC online services
4. Prepare your accounts and computations
5. File using software or HMRC's service

**Late filing penalties**

| How late | Penalty |
|---|---|
| 1 day | £200 |
| 3 months | Another £200 |
| 6 months | 10% of unpaid tax |
| 12 months | Another 10% of unpaid tax |

Penalties apply even if no tax is due. If your return is late three times in a row, the £200 penalties rise to £1,000 each.

**Common mistakes to avoid**

- Filing accounts profit without tax adjustments
- Forgetting to claim capital allowances
- Missing the payment deadline (different from filing deadline)
- Not filing a return for dormant periods
- Claiming personal expenses through the company

Source: HMRC Corporation Tax Return Guidance

## Real-world examples

### Late Filing Penalties

Your company's accounting period ended on 31st December 2024. You must file your CT600 and pay your Corporation Tax by 31st December 2025. Failing to meet this deadline will result in automatic penalties from HMRC, starting with an initial £200 fine.

### Research and Development (R&D) Relief

Your limited company spent £50,000 on qualifying R&D activities. This expenditure needs to be accurately reflected in your tax computations and CT600 to claim the appropriate R&D tax relief, potentially reducing your Corporation Tax liability or even generating a payable tax credit.

### Capital Allowances on Equipment

Your company purchased a new machine for £10,000. You can claim capital allowances on this purchase, deducting a portion of the cost from your taxable profits. This claim must be accurately reflected in your tax computations attached to your CT600 return.

## Common mistakes to avoid

- Forgetting to include all necessary supporting documents like detailed tax computations, which can lead to delays and further scrutiny from HMRC.
- Using the incorrect accounting period dates on the CT600, resulting in the return being rejected or assessed incorrectly.
- Failing to reconcile the figures in the CT600 with the company's statutory accounts, creating inconsistencies that trigger HMRC investigations.
- Not declaring all sources of income, including investment income or gains from the sale of assets, which is a common oversight leading to penalties.

## Frequently asked questions

### What happens if I need to amend my CT600 after I've already submitted it?

You can amend your CT600 return online through HMRC’s online services. There is a time limit for amendments, generally 12 months from the original filing deadline. Make sure to keep records of the changes made and the reasons for the amendment.

### How long should I keep records related to my CT600 return?

You must keep all records relating to your company's Corporation Tax return for at least six years from the end of the accounting period. This includes invoices, bank statements, payroll records, and tax computations, as HMRC can request these for review.

### Can I get an extension to file my CT600 if I have a valid reason?

HMRC is unlikely to grant extensions for filing CT600 returns, even with valid reasons. It's crucial to file on time to avoid penalties. If you're facing genuine difficulties, contact HMRC as soon as possible to discuss your situation.

### What are the penalties for inaccuracies in my CT600 return?

Penalties for inaccuracies depend on whether the error was careless, deliberate, or deliberate and concealed. Penalties can range from a percentage of the underpaid tax to potentially more severe consequences for deliberate misreporting. Ensure accuracy to avoid penalties.

## Practical tips

- Use accounting software that integrates directly with HMRC to submit your CT600 online, reducing the risk of manual errors and simplifying the filing process.
- Review your CT600 and tax computations thoroughly before submission, ideally by another person within the company or an external accountant, to catch any errors or omissions.
- Keep a clear audit trail of all transactions and adjustments made during the tax computation process; this will be invaluable if HMRC queries your return.
- Plan ahead and start preparing your CT600 well in advance of the deadline to avoid last-minute rushes and potential mistakes. Set reminders and allocate sufficient time for accurate completion.

## Related questions

- [When is Corporation Tax due?](https://accounts-os.com/questions/when-is-corporation-tax-due)
- [What is the Corporation Tax rate in 2025?](https://accounts-os.com/questions/what-is-the-corporation-tax-rate-2025)
- [What expenses reduce Corporation Tax?](https://accounts-os.com/questions/what-expenses-reduce-corporation-tax)