The Company Tax Return (CT600): what it is and what goes in it

A Company Tax Return is how a UK limited company tells HMRC its profit or loss for Corporation Tax and works out its Corporation Tax bill. The main form is the CT600.

The profit or loss for Corporation Tax is different from the profit or loss in the company's annual accounts. That is why a return comes with computations: they show how the figures in the return were worked out from the figures in the accounts.

A company must file a return when HMRC sends it a notice to deliver one. It must still file when it made a loss or has no Corporation Tax to pay. A company that is not doing business is usually dormant for Corporation Tax, and the dormant companies guide covers that case.

A return covers one accounting period for Corporation Tax. That is normally the same as the company's financial year, and it can never be longer than 12 months. Accounts that cover more than 12 months need two returns, which is common in a company's first year: the guide to first accounts and the first return has GOV.UK's example.

• The CT600

  The form itself. It holds the company's details, the figures that make up its profit and its tax, and a declaration at the end.

• The accounts

  The company's statutory accounts for the period, as iXBRL. Box 80 says the accounts are for the period the return covers, and box 85 says they are for a different period.

• The computations

  The workings that show how the return's figures were calculated from the accounts, also as iXBRL. GOV.UK asks for everything needed to explain the figures to be in the single computations file. When accounts and computations are not sent, box 90 is where the reasons go.

• Supplementary pages

  Extra pages for particular situations, such as the CT600A for loans to participators by a close company. They are part of the return, and its declaration covers them.

Some boxes matter more to a company that lets property than to others. UK property business losses, for example, have their own box (805), and the company's computations should show how each figure was reached. GOV.UK's guide to completing the return explains every box.

• Starting

  A company should add Corporation Tax services to its HMRC business tax account when it starts to do business, which includes buying, selling, advertising, renting a property and employing someone. HMRC then posts an activation code, instructions and the payment deadline to the registered office, with a letter giving the dates of the first accounting period.

• Paying

  Corporation Tax is due nine months and one day after the accounting period ends, for taxable profits up to £1.5 million. Above that it is paid in instalments. When that day is a weekend or bank holiday, the money has to reach HMRC on the last working day before it, unless it is paid by Faster Payments.

• Filing

  The return is due 12 months after the accounting period ends, or three months after HMRC's notice to deliver the return was served if that is later (Finance Act 1998, Schedule 18, paragraph 14).

The payment date comes first: it is about three months before the return is due. The accounts for Companies House have a date of their own, nine months after the financial year ends. The filing deadlines page sets out how each date is worked out.

A Company Tax Return is filed online. A company can get an accountant to prepare and file it, or do it itself with commercial software. Paper is only for a company with a reasonable excuse for not filing online, or one filing in Welsh. HMRC's own online service for the accounts and the return closed on 31 March 2026.

HMRC publishes the list of commercial software suppliers it takes returns from. The guide to what to look for in CT600 software sets out what to ask before choosing one.

The return ends with a declaration: the name, the date and the status of the person making it. It covers the supplementary pages too, and GOV.UK's warnings about prosecution, and about late and incorrect returns, apply to them. Whoever makes the declaration should read the whole return first.

• Late filing: £200 from the first day and another £200 after three months. At six months HMRC estimates the Corporation Tax and adds a penalty of 10% of the unpaid tax, and another 10% at twelve months. If a return is late three times in a row, the £200 penalties become £1,000 each. A return that is late is penalised even when there is no tax to pay.
• Late payment: HMRC may charge interest on tax that is not paid on time, and pays interest on tax paid early.
• Appeals: a late filing penalty can be appealed online when there was a reasonable excuse, and the return has to be filed before the appeal.
• Changes: a change to a return is called an amendment, and it must usually be made within 12 months of the filing deadline. After that, overpaid tax is claimed through overpayment relief, and underpaid tax is reported through HMRC's online disclosure service. HMRC may charge a penalty for errors.

This service is for a UK limited company that lets property. It turns reviewed bank statements into double-entry books, works out Corporation Tax for each accounting period, and puts the CT600 values, the micro-entity accounts and the computations into one filing pack.

This service does not file anything. An owner approves the filing pack, and you, or your accountant, file the accounts and the return, with a step-by-step guide to follow. The filing by hand page has the detail.

A qualified accountant has not reviewed the calculations. Treat the output as a working draft for you or your accountant to check.