CT600: Everything You Need to Know

The CT600 is the Corporation Tax Return that every UK limited company must file with HMRC. Whether your company made a profit, a loss, or had no activity at all — you still need to file one. This guide covers everything you need to know.

What Is the CT600?

The CT600 is the official form used to report your company's taxable profits (or losses) to HMRC. It is used to calculate the amount of corporation tax your company owes for each accounting period.

Think of it as your company's annual tax return — similar to a Self Assessment return for individuals, but for companies.

Who Needs to File a CT600?

Every UK limited company must file a CT600, including:

• Active trading companies
• Dormant companies (those with no transactions)
• Companies that made a loss
• Companies with no corporation tax to pay

The only exception is if HMRC has confirmed your company is dormant and does not need to file.

When Is the CT600 Due?

You must file your CT600 within 12 months of the end of your company's accounting period. However, any corporation tax owed must be paid within 9 months and 1 day of the accounting period end.

For example, if your accounting period ends on 31 March 2024:

• Corporation tax payment deadline: 1 January 2025
• CT600 filing deadline: 31 March 2025

Late Filing Penalties

HMRC takes late filing seriously:

• 1 day late: £100 penalty
• 3 months late: additional £100 penalty
• 6 months late: HMRC estimates your tax and adds 10% of the unpaid amount
• 12 months late: additional 10% of unpaid tax

These penalties are separate from interest charges on late tax payments.

What Information Do You Need?

To complete a CT600, you will need:

1. Company details — name, UTR number, Companies House registration number
2. Accounting period — start and end dates
3. Turnover — total income/revenue
4. Cost of sales and expenses — broken down by category
5. Capital allowances — claims for qualifying asset purchases
6. Profit/loss — the taxable profit or loss figure
7. Tax computation — calculation of corporation tax owed
8. Statutory accounts — these must accompany your CT600

You will also need to attach your tax computation (sometimes called a CT600 computation), which shows how you arrived at the taxable profit figure.

Common Sections of the CT600

Box 145: Turnover

Your total income before any deductions.

Box 155: Trading Profits

Profits from your company's trade after deducting allowable expenses.

Box 235: Total Profits

The total taxable profit after all adjustments.

Box 440: Tax Chargeable

The corporation tax amount calculated on your profits.

Box 470: Tax Payable

The final amount of tax you need to pay after any deductions or credits.

Common Mistakes to Avoid

1. Wrong accounting period dates — ensure they match Companies House records
2. Forgetting capital allowances — these can significantly reduce your tax bill
3. Not including all income — bank interest, rental income, and other sources count
4. Missing the payment deadline — remember, tax is due before the filing deadline
5. Not filing when dormant — even dormant companies usually need to file

Filing Methods

You can file your CT600:

• Online via HMRC's service — mandatory for most companies
• Through commercial software — accounting packages that submit directly to HMRC
• Via an accountant — who files on your behalf
• Using TaxDocs — our service generates CT600-ready tax computations

Key Takeaways

• The CT600 is mandatory for all UK limited companies
• File within 12 months of your accounting period end
• Pay corporation tax within 9 months and 1 day
• Late filing and late payment incur separate penalties
• Accuracy is crucial — mistakes can trigger HMRC enquiries
• Keep detailed records to make the process easier each year

This article is for informational purposes only and does not constitute tax advice.