The CT600 corporation tax return is the document every active UK limited company must file with HMRC each year. It comes bundled with an iXBRL-tagged set of accounts and a tax computation that bridges your accounting profit to your taxable profit. Get any one of those three components wrong and HMRC will reject the bundle. This guide sits within our wider UK tax and VAT hub, where you will find the surrounding rules referenced here.
What goes in the CT600 package
A complete corporation tax submission is three files joined together:
- The CT600 form itself (with supplementary pages CT600A-CT600N as needed)
- Statutory accounts in iXBRL format
- The tax computation in iXBRL format showing add-backs, deductions, capital allowances and reliefs
You file via HMRC’s Corporation Tax Online service or, more commonly, through commercial software that produces all three files together. If you are new to the tax itself, start with our overview of Corporation Tax before tackling the return.
Key dates
| Event | Deadline |
|---|---|
| Pay corporation tax | 9 months and 1 day after year end |
| File CT600 + accounts + computation | 12 months after year end |
| File accounts at Companies House | 9 months after year end |
| Notify chargeability (first year) | 3 months after start of trade |
| Pay tax in instalments (large companies) | Quarterly during the period |
Note that payment is due before filing. Companies routinely file in month 11 but must pay in month 9 plus one day, working from an estimated computation if the accounts are not yet finalised. For the full timetable, including instalment dates for larger companies, see our guide to corporation tax payment deadlines.
Corporation tax rates
| Profit band | Rate |
|---|---|
| Up to the lower limit | Small profits rate |
| Between the lower and upper limits | Marginal relief applies |
| Above the upper limit | Main rate |
Marginal relief smooths the transition between the small profits rate and the main rate, giving an effective rate higher than either band over the middle range of profits. Associated companies share the profit limits, so a group of three companies divides each limit three ways, which can push more of each company’s profit into the marginal band. Always check the current rate and limits before you compute the charge, as these are set each financial year.
Building the tax computation
The computation starts from accounting profit and adjusts:
- Add back depreciation, amortisation, entertaining, fines and donations to political parties
- Deduct capital allowances (AIA, FYA, writing-down allowances)
- Add or deduct chargeable gains and losses
- Apply trading loss reliefs and group relief
- Subtract R&D enhanced deductions or RDEC credits
The result is your taxable trading profit. See our capital allowances for UK businesses article for the asset rules and R&D tax credits for SMEs for the innovation reliefs.
Common errors that trigger HMRC enquiries
- Tagging accounts with iXBRL errors that fail validation
- Claiming AIA on cars (use FYA for low-emission vehicles instead)
- Missing the associated company count for marginal relief
- Forgetting director’s loan account section 455 tax (CT600A)
- Reporting dividends paid as an expense
- Using the wrong accounting period when it spans two financial years
Any of these can turn a routine submission into an enquiry. Our guide to an HMRC tax investigation explains what to expect and how to keep the records that head one off.
Late filing and payment
| Delay | Penalty |
|---|---|
| 1 day late | £100 |
| 3 months late | Further £100 |
| 6 months late | 10% of unpaid tax |
| 12 months late | Further 10% of unpaid tax |
| Late payment | Daily interest at HMRC official rate |
Two consecutive late filings increase the £100 penalties to £500.
Tying it together
Get your year-end ready first. Walk through the year-end accounting checklist for UK limited companies and read the line-by-line CT600 guide for detailed help. The HMRC Corporation Tax for Company Tax Return guidance is the authoritative source for current forms. Join the Nexbal beta to keep the year’s invoices, receipts and bank postings ready for your accountant.