How to File Your Company Tax Return

Every UK limited company must file a Company Tax Return (form CT600) with HMRC, even if the company made no profit or had no trading activity during the period. This is separate from the annual accounts you file at Companies House.

Getting it wrong – or filing late – triggers automatic penalties and interest charges. Here is how to do it properly.

What is the CT600?

The CT600 is the form HMRC uses to assess your company’s Corporation Tax liability. It contains:

• Your company’s taxable income and gains
• Capital allowances claimed
• Any reliefs or deductions (R&D credits, group relief, losses)
• The amount of Corporation Tax due

You do not just send the form on its own. You also need to attach:

• Your statutory accounts in iXBRL (inline eXtensible Business Reporting Language) format
• A Corporation Tax computation showing how you arrived at the taxable profit

Key deadlines

The deadlines are based on your company’s accounting period, not the standard tax year:

These are two different deadlines. You need to pay your tax well before you need to file the return. Many business owners confuse the two.

For example, if your accounting period runs from 1 April 2024 to 31 March 2025:

• Payment deadline: 1 January 2026
• Filing deadline: 31 March 2026

If your company’s taxable profits exceed £1.5 million, you must pay in quarterly instalments instead. This is called the quarterly instalment payment (QIP) regime.

What you need before you start

Gather everything before you begin the filing process. You will need:

Financial records

• Finalised profit and loss account for the period
• Balance sheet at the period end date
• Bank statements covering the full period
• Records of all income, expenses and transactions
• Details of any dividends paid during the period

Tax-specific information

• Capital allowances computation (AIA, writing-down allowances, structures and buildings allowance)
• Details of any losses carried forward from earlier periods
• R&D tax credit calculations if applicable
• Records of any related party transactions
• Details of any company cars or benefits in kind

Company details

• Your company’s UTR (Unique Taxpayer Reference) – a 10-digit number from HMRC
• Your Companies House registration number
• Your company’s accounting reference date
• Registered office address

Step-by-step filing process

Step 1: Prepare your accounts

Your accounts must comply with UK GAAP (usually FRS 102 or FRS 105 for micro-entities). If you use an accountant, they will prepare these for you. If you prepare them yourself, your accounting software may generate them in the correct format.

Step 2: Convert accounts to iXBRL

HMRC requires your accounts in iXBRL format. This is a structured data format that allows HMRC’s systems to read and process your accounts automatically.

Most accounting software and filing tools handle this conversion for you. If you need to do it manually, HMRC provides a list of approved iXBRL tagging software on their website.

Step 3: Prepare your Corporation Tax computation

The tax computation bridges the gap between your accounting profit and your taxable profit. The main adjustments are:

Your taxable profit is what the Corporation Tax rate applies to.

Step 4: Calculate Corporation Tax

Apply the correct Corporation Tax rate to your taxable profit:

If your company has any associated companies (companies under common control), the thresholds are divided by the number of associated companies plus one.

Step 5: File online

You must file your CT600 online. There are two main ways:

HMRC’s own service:

• Log in to your HMRC business tax account
• Use the Corporation Tax online filing service
• Enter the CT600 data manually
• Attach your iXBRL accounts and computations

Commercial software:

• Most accounting packages can file directly with HMRC
• This is generally faster and reduces errors since the data comes from your existing records
• The software handles the iXBRL conversion automatically

Step 6: Pay your Corporation Tax

Pay using one of these methods:

• Direct Debit – set up through HMRC’s online service
• Online banking – pay to HMRC’s Cumbernauld bank account using your payment reference
• CHAPS – same-day payment through your bank
• BACS – takes three working days

Always use the correct payment reference, which is your 17-character Corporation Tax reference number. If you use the wrong reference, your payment may not be allocated correctly and you could face late payment interest.

Common mistakes to avoid

Incorrect accounting period

Your CT600 covers a specific accounting period, which may not be the same as your financial year. A company’s accounting period cannot be longer than 12 months. If your financial year is longer (for example, an 18-month first period), you need to file two CT600s.

Forgetting disallowable expenses

Not all expenses in your accounts are deductible for tax purposes. Common items that must be added back include:

• Client entertaining
• Depreciation (replaced by capital allowances)
• Some legal fees (e.g. related to capital items)
• General provisions for bad debts (only specific bad debts are allowable)
• Fines for breaking the law

Not claiming all capital allowances

Many companies claim less than they are entitled to. Make sure you have considered:

• Annual Investment Allowance (£1,000,000 per year for qualifying assets)
• Writing-down allowances (18% main rate pool, 6% special rate pool)
• Full expensing for qualifying main rate plant and machinery
• Structures and buildings allowance (3% straight-line for commercial property)

Missing the payment deadline

Remember: the payment deadline is three months before the filing deadline. If you wait until the filing deadline to pay, you will already owe interest on three months of late payment.

Penalties for late filing

HMRC imposes automatic penalties for late CT600 filing:

If your return is late three times in a row, the £100 penalties increase to £500 each.

On top of the filing penalties, HMRC charges interest on any Corporation Tax paid late. The current rate is published on the HMRC website and changes periodically.

Making Tax Digital for Corporation Tax

HMRC is extending Making Tax Digital (MTD) to Corporation Tax. While the timeline has been pushed back several times, companies should prepare by:

• Using MTD-compatible software for their accounting records
• Keeping digital records of all transactions
• Being ready to submit quarterly updates when required

Using software that already supports MTD for VAT and is built for digital filing will make the transition smoother. Check whether your current software is ready or explore alternatives that are built for digital-first filing .

After you file

Once HMRC processes your return, they may:

• Accept it without further queries
• Open an enquiry (they have 12 months from the filing date to do this)
• Issue a discovery assessment if they believe the return is incorrect

Keep all your records for at least six years after the end of the accounting period. If HMRC opens an enquiry, you will need to provide supporting documentation for everything in your return.

For a broader look at what needs to happen around your year end, see the year-end accounting checklist .