Corporation Tax Return (CT600): Complete Filing Guide

Every UK limited company must file a Corporation Tax return (CT600) with HMRC, even if the company made no profit or owes no tax. The CT600 is how you report your company's taxable profits, calculate the tax due, and claim any reliefs or allowances. This guide covers everything you need to know about filing your CT600 corporation tax return UK, from what you need to prepare to how to avoid costly penalties.

Key Deadlines: Filing vs Payment

Before diving into the details, understand this critical distinction: the filing deadline and payment deadline are different.

This means you must pay your Corporation Tax approximately 3 months before you submit your CT600 return. You'll need to estimate your tax liability if final figures aren't ready.

Example for year-end 31 March 2025:

• Payment deadline: 1 January 2026 (9 months and 1 day)
• Filing deadline: 31 March 2026 (12 months)

For comprehensive deadline information, see our corporation tax deadline guide.

Timeline Example: Year-End 31 December 2024

To illustrate how these deadlines work in practice:

1 January 2024 - 31 December 2024 Your accounting period (the trading year you're reporting on)

31 March 2025 HMRC sends a notice to file your CT600 return (within 3 months of period end)

1 October 2025 PAYMENT DEADLINE - Corporation Tax must be paid to HMRC

31 December 2025 FILING DEADLINE - CT600 return, accounts, and computations must be submitted

Note how payment is due before filing. This means you need to calculate (or estimate) your tax liability before submitting the detailed return.

What Is the CT600 and Who Must File?

The CT600 is HMRC's official Corporation Tax Return form. It's the document that tells HMRC:

• How much profit your company made during the accounting period
• What adjustments you've made to arrive at taxable profit
• How much Corporation Tax you owe
• What reliefs or allowances you're claiming

Who must file a CT600?

Every UK limited company must file a CT600, including:

• Active trading companies (regardless of profit or loss)
• Dormant companies (unless formally exempt)
• Companies with no tax to pay
• Loss-making companies
• Companies claiming refunds or reliefs

The only exception is companies that have been formally notified by HMRC that they're dormant for Corporation Tax purposes. Even then, you must notify HMRC if the company becomes active again.

What You Need to File a CT600

Filing a CT600 requires three main components:

1. Company Accounts

Your statutory accounts must accompany the CT600 submission. The format depends on your company size:

Micro-entity accounts (turnover under £632,000, balance sheet under £316,000, 10 or fewer employees):

• Simplified balance sheet
• No profit and loss account required for Companies House
• Minimal notes

Small company accounts (turnover under £10.2 million, balance sheet under £5.1 million, 50 or fewer employees):

• Abbreviated balance sheet
• Profit and loss account
• Basic notes to accounts

Full accounts (medium and large companies):

• Detailed balance sheet and profit and loss account
• Directors' report
• Comprehensive notes
• Potentially auditor's report

Most UK limited companies qualify as small or micro-entities. See our limited company accounting requirements guide for detailed information on account formats.

2. The CT600 Form Itself

The CT600 form captures:

• Company identification and accounting period details
• Trading profits and losses
• Income from investments and property
• Chargeable gains
• Reliefs claimed (R&D credits, capital allowances, losses)
• Corporation Tax calculation
• Declaration by a company officer

3. Tax Computations

Computations are the bridge between your accounting profit and taxable profit. They show HMRC exactly how you calculated the Corporation Tax due, including all adjustments made to your accounting figures.

How Corporation Tax Is Calculated

Corporation Tax isn't simply applied to your accounting profit. You must make adjustments to arrive at your taxable profit.

Step 1: Start with Accounting Profit

Your profit and loss account shows your accounting profit (or loss) for the year. This is revenue minus expenses, calculated according to UK accounting standards.

Step 2: Add Back Disallowable Expenses

Certain expenses that reduce accounting profit aren't allowable for Corporation Tax purposes. You must add these back when calculating taxable profit:

Common disallowable expenses:

• Client and staff entertaining (but not staff-only events up to £150 per head)
• Depreciation (replaced by capital allowances)
• Fines and penalties
• Political donations
• Non-business expenses
• Provisions for future costs (unless specific and certain)
• Legal costs related to capital items

Step 3: Deduct Capital Allowances

While depreciation is disallowable, you can claim capital allowances on qualifying business assets:

• Annual Investment Allowance (AIA): 100% deduction on the first £1 million of qualifying plant and machinery
• Full expensing: 100% first-year allowance on qualifying main rate plant and machinery
• Writing Down Allowance: 18% (main rate) or 6% (special rate) on remaining pool balances
• Structures and Buildings Allowance: 3% annual allowance on qualifying commercial structures

Step 4: Apply Trading Losses

If you have losses brought forward from previous years, you can offset these against current profits, reducing your tax liability. Loss relief rules are complex, with different treatments for trading losses, capital losses, and non-trading deficits.

Step 5: Calculate Tax at the Correct Rate

Apply the appropriate Corporation Tax rate to your taxable profit:

Use our corporation tax calculator to see your exact liability with marginal relief applied.

Important: These thresholds are divided by the number of associated companies plus one. If you control multiple companies, your thresholds will be lower.

Step 6: Deduct Reliefs and Credits

Finally, deduct any applicable tax reliefs:

• R&D tax credits
• Creative industry reliefs
• Patent box relief
• Double taxation relief (if you've paid foreign tax)

What Are Tax Computations and Why Are They Needed?

Tax computations are a detailed working document that reconciles your accounting profit to taxable profit. They're required because HMRC needs to see exactly how you arrived at the Corporation Tax figure on your CT600.

Computations typically include:

• Profit per accounts (starting point)
• Additions to profit (disallowable expenses)
• Deductions from profit (capital allowances, reliefs)
• Taxable profit calculation
• Corporation Tax calculation with rate applied
• Any reliefs reducing the tax due
• Final Corporation Tax liability

Think of computations as showing your working. The CT600 shows the final answers; computations show how you got there. HMRC may query your return if computations are missing, unclear, or don't reconcile to the figures in your CT600.

Example Computation Layout

A simplified computation might look like this:

Your actual computations will be more detailed, with supporting schedules for capital allowances, loss relief, and any reliefs claimed.

Common CT600 Sections Explained

The CT600 form has multiple sections. Here are the key ones you'll need to complete:

Company Information (Box 1-30)

Basic details including:

• Company name and registration number
• Unique Taxpayer Reference (UTR)
• Accounting period start and end dates
• Type of company and activities

Turnover and Profits (Box 145-185)

• Total turnover for the period
• Trading profit or loss
• Income from property
• Investment income
• Chargeable gains

Corporation Tax Calculation (Box 430-475)

• Profits chargeable to Corporation Tax
• Tax rate applied
• Marginal relief calculation (if applicable)
• Corporation Tax due before reliefs

Reliefs and Deductions (Box 275-395)

• Capital allowances claimed
• Trading losses utilised
• R&D tax credits
• Other reliefs

Tax Payable and Repayable (Box 510-570)

• Final Corporation Tax liability
• Payments already made
• Tax due or refund claimed

Supplementary Pages

Depending on your circumstances, you may need additional schedules:

• CT600A (Loans to participators)
• CT600B (Controlled Foreign Companies)
• CT600C (Group and consortium relief)
• CT600D (Insurance companies)
• CT600E (Charities and community amateur sports clubs)

Most small companies only need the main CT600 form without supplementary pages.

How to File Your CT600

There are three main methods for filing your CT600 corporation tax return UK:

1. HMRC Corporation Tax Online Service

HMRC provides a free online service for filing CT600 returns. It's suitable for straightforward returns and includes:

• Step-by-step form completion
• Inline validation to catch errors
• Direct submission to HMRC
• Confirmation of receipt

Limitations: The HMRC service can be cumbersome for complex returns and doesn't integrate with your accounting records.

2. Commercial Software

Most accounting software packages can file CT600 returns directly with HMRC. This approach:

• Pulls data directly from your accounts
• Reduces manual data entry and errors
• Generates computations automatically
• Keeps an audit trail of all submissions

Popular options: Xero, FreeAgent, Sage, QuickBooks, and specialist Corporation Tax software.

3. Through Your Accountant

Many company directors engage an accountant to prepare and file their CT600. Your accountant will:

• Prepare annual accounts
• Calculate tax adjustments and allowances
• Complete computations
• File the CT600 on your behalf
• Handle any HMRC queries

Even if using an accountant, you as director remain legally responsible for the accuracy of the return.

Filing Requirements: iXBRL Format

HMRC requires CT600 returns to be submitted in iXBRL (Inline eXtensible Business Reporting Language) format. This is a machine-readable format that allows HMRC to process returns automatically.

You don't need to understand iXBRL yourself. Commercial software and accountants handle the formatting automatically. However, you should be aware that:

• Paper filing is no longer accepted (except in exceptional circumstances)
• Returns must be submitted electronically through approved channels
• The iXBRL format includes both the CT600 form and accounts in a single submission

Late Filing Penalties

HMRC imposes automatic penalties for late CT600 filing, separate from any interest on late payment:

Example: If your company owes £15,000 Corporation Tax and you file 13 months late:

• 1 day penalty: £100
• 3 month penalty: £100
• 6 month penalty: £1,500 (10% of £15,000)
• 12 month penalty: £1,500 (10% of £15,000)
• Total penalties: £3,200

This is in addition to interest on the late payment itself.

Repeated late filing: If you file late for three consecutive accounting periods, the fixed penalties increase to £500 for the third year and beyond.

Late Payment Interest

While there are no fixed penalties for late Corporation Tax payment (unlike filing), you will pay interest from the day after the due date until payment is received.

The late payment interest rate is set by HMRC and currently stands at approximately 7.75% per year. Interest compounds daily, so delays get expensive quickly.

Example: £20,000 tax paid 60 days late would incur approximately £255 in interest charges.

Amending Your CT600 Return

Made a mistake or discovered new information? You can amend your CT600 return within 12 months of the original filing deadline.

How to amend:

1. File an amended CT600 through the same method as the original (online service or software)
2. Explain what's changed and why
3. Include updated computations and accounts if relevant
4. Pay any additional tax due immediately

After the 12-month window: You can no longer amend the return yourself. You must write to HMRC explaining the error and request a correction. HMRC has four years from the end of the accounting period to open an enquiry into your return.

Overpayment claims: If you've overpaid Corporation Tax due to an error, you can claim a refund by amending your return or, if outside the amendment window, by making a formal claim to HMRC.

First-Year Companies: Special Considerations

If this is your company's first accounting period, there are some additional points to note:

Longer first period: Your first accounting period can be up to 18 months from incorporation (though for Corporation Tax purposes, it's split into periods of no more than 12 months each).

Registering for Corporation Tax: You must register for Corporation Tax within 3 months of starting to trade. HMRC usually registers you automatically when you incorporate through Companies House, but verify this in your HMRC online account.

No prior year figures: Your first CT600 won't have comparative figures, which is normal. Focus on accurately reporting the current period.

Brought-forward losses: You won't have any trading losses to bring forward in your first period (unless you've transferred a business into the company).

How AccountsOS Helps with CT600 Filing

Managing Corporation Tax compliance doesn't have to be stressful. AccountsOS automates the heavy lifting so you can focus on running your business.

Deadline tracking: AccountsOS monitors your accounting period and automatically calculates both payment and filing deadlines. You'll receive reminders well in advance, so there's no risk of missing a date.

Real-time tax estimates: Throughout the year, AccountsOS provides running estimates of your Corporation Tax liability based on current trading. Budget effectively and avoid year-end surprises.

Organised records: Every transaction, receipt, and expense is categorised and stored. When CT600 preparation time arrives, all your records are ready and accessible.

AI-powered guidance: Unsure whether an expense is allowable? Ask AccountsOS in plain English and get instant, accurate answers based on current HMRC rules.

See how AccountsOS works or explore how we compare to traditional accountants.

Frequently Asked Questions

What is a CT600 form?

The CT600 is HMRC's Corporation Tax Return form that UK limited companies must file annually. It reports your company's profits, calculates the Corporation Tax due, and declares any reliefs or allowances claimed. The form must be submitted with your company accounts and tax computations.

When is the CT600 filing deadline?

The CT600 must be filed within 12 months of the end of your accounting period. For a company with a 31 March 2025 year-end, the filing deadline is 31 March 2026. Note that the payment deadline is earlier, at 9 months and 1 day after the accounting period ends.

Can I file my CT600 myself without an accountant?

Yes, you can file your own CT600 using HMRC's free online service or commercial software. However, Corporation Tax rules are complex, and errors can be costly. Many directors engage an accountant at least for their first few returns or when circumstances are complex (capital allowances, R&D claims, loss relief).

What happens if I miss the CT600 deadline?

You'll face automatic penalties: £100 if one day late, an additional £100 at 3 months, then 10% of unpaid tax at 6 months and another 10% at 12 months. These penalties apply even if you've already paid your Corporation Tax. File as soon as possible to minimise penalties.

Do I need to file a CT600 if my company made a loss?

Yes. Every UK limited company must file a CT600 for each accounting period, regardless of whether it made a profit or loss. Filing a loss-making return is important because you can carry the loss forward (or back) to offset against profits in other years, potentially claiming a tax refund.

What's the difference between the CT600 and annual accounts?

Annual accounts are financial statements (balance sheet, profit and loss) filed with Companies House showing your company's financial position. The CT600 is filed with HMRC and calculates your Corporation Tax liability. Both use similar underlying data but serve different purposes and have different deadlines (9 months for accounts, 12 months for CT600).

How do I pay my Corporation Tax?

Corporation Tax can be paid via direct debit, bank transfer (CHAPS or Faster Payments), or debit/corporate credit card through your HMRC online account. You'll need your unique 17-character payment reference starting with your company number. Ensure payment reaches HMRC by the deadline to avoid interest charges.

Can I claim R&D tax credits on my CT600?

Yes, if your company undertakes qualifying research and development activities. R&D tax credits can significantly reduce your Corporation Tax bill or provide a cash refund for loss-making companies. The claim is made as part of your CT600 return, with supporting documentation on your R&D projects. The relief is complex, and many companies use specialist advisers to maximise claims.

Don't Let CT600 Filing Stress You Out

Filing your CT600 corporation tax return UK correctly and on time is essential for staying compliant with HMRC. Between payment deadlines, filing deadlines, computations, and penalty structures, there's a lot to track.

AccountsOS takes the stress out of Corporation Tax compliance. With automated deadline tracking, real-time tax estimates, and AI-powered guidance, you'll always know where you stand. Focus on growing your business while AccountsOS handles the complexity.

Ready to simplify your company tax? See how AccountsOS works and experience accounting that works as hard as you do.