How to File Your Company Tax Return

Every limited company that receives a notice from HMRC must file a Company Tax Return (form CT600), even if it made a loss or has no tax to pay. You need to file online using commercial software, calculate your Corporation Tax bill, and meet strict deadlines to avoid penalties. This guide walks you through the process step by step.

Who needs to file a Company Tax Return

You must file a Company Tax Return if your company or association receives a 'notice to deliver a Company Tax Return' from HMRC. This applies even if your company:

• Made a loss during the accounting period
• Has no Corporation Tax to pay

If you're self-employed as a sole trader or in a partnership, you don't file a Company Tax Return. Instead, you complete a Self Assessment tax return.

What you need to include

When you file your Company Tax Return, you must:

• Calculate your profit or loss for Corporation Tax purposes (this differs from the profit or loss in your annual accounts)
• Work out your Corporation Tax bill
• Include a computation showing how you arrived at these figures

You can either hire an accountant to prepare and file your return, or do it yourself using commercial software.

How to file your return

You must file your Company Tax Return online using approved commercial software. HMRC provides a list of commercial software suppliers that can generate the correct format, including the required XBRL tagging.

You can only file a paper return if you:

• Have a reasonable excuse (such as serious illness or a death in the family)
• Are filing in Welsh

The previous HMRC online service for filing accounts and tax returns has closed, so you'll need to use commercial software instead.

Filing deadlines

Your Company Tax Return must be filed within 12 months of the end of the accounting period it covers. An accounting period is the period your Corporation Tax is calculated on, which is usually the same as your company's financial year.

For example, if your accounting period ends on 31 March 2025, your filing deadline is 31 March 2026.

Important: The deadline for paying your Corporation Tax bill is different. You must pay your Corporation Tax bill within 9 months and one day after the end of the accounting period.

Penalties for missing the deadline

HMRC charges automatic penalties if you file late:

• 1 day late: £200 penalty
• 3 months late: Another £200 penalty (total £400)
• 6 months late: HMRC estimates your Corporation Tax bill and adds a 10% penalty on any unpaid tax
• 12 months late: Another 10% penalty on any unpaid tax

If your tax return is late three times in a row, the £200 penalties increase to £1,000 each.

What happens if your return is over 6 months late

When your return reaches 6 months late, HMRC will write to you with a 'tax determination' – their estimate of how much Corporation Tax you owe. You cannot appeal against a tax determination.

You must still:

• Pay the Corporation Tax shown in the determination
• File your actual Company Tax Return

Once you file, HMRC will recalculate the correct amount of interest and penalties.

Appealing against penalties

You can appeal against a late filing penalty if you have a reasonable excuse for missing the deadline. Reasonable excuses include situations like serious illness, a death affecting your ability to file, or unexpected technology failures.

You must file your Company Tax Return before you can appeal a late filing penalty.

To appeal, you need:

• Your company's Unique Taxpayer Reference (UTR)
• The date shown on the penalty notice
• The penalty amount
• The end date of the accounting period the penalty relates to
• An explanation of why you couldn't file by the deadline

You must complete the online appeal form in one go – you cannot save it and return later. After completing it, print the form and send it to the address shown.

Making changes to your return

You can amend your Company Tax Return within 12 months of the filing deadline. After that, the time limit expires.

To make changes within this 12-month window, either:

• Use commercial software to submit an amended return
• Write to HMRC's Corporation Tax office or send a paper return

HMRC may check your return for errors and can charge penalties if they find mistakes.

Changes after the 12-month deadline

If you discover you've overpaid Corporation Tax after the 12-month amendment window has closed, you may still be able to claim it back using overpayment relief.

If you've underpaid Corporation Tax, you should tell HMRC using their online disclosure service as soon as possible.

Current issues with the online service

HMRC has announced several changes to the Corporation Tax online service that affect returns filed in 2025/26 and beyond:

New 40% first-year allowance: From 1 January 2026, a new 40% first-year allowance applies to qualifying plant and machinery. The online service won't be updated until April 2027, so if you need to claim this relief before then, use the existing capital allowance boxes (boxes 725 or 750 for claim amounts, box 760 for qualifying expenditure).

Zero-emission cars: The 100% first-year allowance for zero-emission cars has been extended to 31 March 2027. If your accounting period starts on or after 1 April 2026 and you're filing before April 2027, use boxes 725 and 750 to report your claim instead of boxes 726 and 751.

Loans to participators: From 6 April 2026, the tax rate on loans to participators increases from 33.75% to 35.75%. The online service won't reflect this change until 6 April 2027. If this applies to you and you file before then, you'll need to amend your return after 6 April 2027.

Additional requirements for certain claims

If you're claiming Research and Development (R&D) tax relief or expenditure credit, you must submit a separate form called 'Submit information to support your Research and Development (R&D) claim' alongside your Company Tax Return.

Similarly, if you're claiming creative industry tax reliefs or credits, you must submit the 'Support your claim for creative industry tax reliefs' form and include the supplementary page CT600P with your return.

Sources

• Company Tax Returns
• File your accounts and Company Tax Return
• Changes and issues affecting the Corporation Tax online service

This article provides general guidance based on current HMRC rules. For advice specific to your situation, speak to your accountant.