What a CT600 is, who has to file one, the deadlines, and how it differs from the accounts you file at Companies House.
A CT600 is the Company Tax Return that a limited company files with HMRC. It reports the company’s profit and works out how much Corporation Tax is due. Almost every active UK company has to file one every year. This guide explains what the CT600 is, who files it, the deadlines, and how it differs from your Companies House accounts.
What the CT600 is for
The CT600 tells HMRC how much profit your company made and how much Corporation Tax it owes. It is submitted alongside a set of accounts and a tax computation that shows how you got from the profit in the accounts to the taxable profit. Together these make up your Company Tax Return.
Who has to file one
Any company that is active for Corporation Tax must file a CT600, even if it made a loss or owes no tax. A company that is dormant for Corporation Tax and has told HMRC usually does not have to file one. If HMRC has sent a notice to deliver a return, you must respond to it either way.
The deadlines
There are two separate dates and people often mix them up:
- You must file the CT600 within 12 months of the end of your accounting period
- You must pay any Corporation Tax within 9 months and 1 day of the end of your accounting period
So the tax is due before the return has to be filed. Plan for the earlier payment date, not the later filing date.
CT600 versus Companies House accounts
This is the single most common source of confusion. The CT600 and its accounts go to HMRC. A separate set of accounts goes to Companies House. They can be based on the same figures but they are two different filings, sent to two different bodies, with two different deadlines. Our guide on Companies House versus HMRC accounts sets out the full picture.
How the CT600 is filed
The CT600 is filed online, either through HMRC’s free service for simple small companies or through commercial software. It is always submitted in iXBRL, the machine-readable format HMRC requires. There is no paper CT600 for ordinary companies.
The 2028 change and the CT600
The 2028 filing reform is about Companies House accounts moving to software only. HMRC filing, including the CT600, already works this way and does not change in 2028. If you file both, the part that changes is your Companies House accounts, and we are building a free tool to make that side simple.