CT600 guidance: the company tax return explained

The CT600 is the company tax return that every UK limited company files with HMRC to report its taxable profits and work out how much corporation tax it owes. This guide explains who has to file, what goes into the return, the deadlines and penalties, and the mistakes we see most often.

A limited company must file a CT600 for each accounting period once HMRC sends a notice to deliver a company tax return. That usually happens shortly after a company starts trading. A company that is dormant can usually tell HMRC so rather than filing, but it must still file its accounts at Companies House.

The return is filed online and must include: - The CT600 form itself, showing the company's profits, reliefs and the tax due - The company's statutory accounts - The corporation tax computation, which adjusts the accounting profit to taxable profit - Any supplementary pages that apply, such as CT600A for loans to directors and other participators, or CT600L for R&D tax relief

The accounts and computation are submitted in a tagged format called iXBRL, which is why most companies use accounting software or an accountant to file.

The starting point is the profit in the accounts. The computation then makes adjustments, for example: - Adding back depreciation and claiming capital allowances instead - Adding back costs that are not allowed for tax, such as client entertaining - Deducting reliefs, such as R&D tax relief or losses brought forward

Currently, companies with profits up to £50,000 pay the small profits rate of 19%. Companies with profits over £250,000 pay the main rate of 25%. Between those limits, marginal relief applies, so the effective rate sits between the two. The limits are divided between associated companies and reduced for accounting periods shorter than 12 months.

- **Payment:** corporation tax is usually due 9 months and 1 day after the end of the accounting period. Large companies pay by quarterly instalments instead. - **Filing:** the CT600 must be filed within 12 months of the end of the accounting period.

An accounting period for corporation tax cannot be longer than 12 months. If your accounts cover a longer period, for example your first accounts after incorporation, you will need two tax returns for that one set of accounts.

HMRC charges a £100 penalty if the return is even one day late, and another £100 if it is three months late. If it is six months late, HMRC can estimate the tax and add a penalty of 10% of the unpaid tax, with a further 10% at 12 months. Penalties increase for repeated late filing, and interest is charged on tax paid late.

- Filing a single return for a period of account longer than 12 months - Missing the CT600A when a director's loan account is overdrawn at the year end. Tax at 33.75% can be due on the loan if it is not repaid within 9 months of the year end. - Not counting associated companies when working out which rate applies - Claiming capital allowances on items that do not qualify, or missing ones that do - Leaving it until the filing deadline, three months after the tax was due to be paid

We prepare your accounts, corporation tax computation and CT600 together, check which reliefs and allowances apply, and tell you what is due and when well before the payment deadline. See our [accounts and tax service](/accounts-and-tax), or our [R&D tax relief](/rd-tax-relief) support if you are claiming for innovation.

For HMRC's own guidance, see [Company Tax Returns on GOV.UK](https://www.gov.uk/company-tax-returns).