Filing your CT600 corporation tax return
How to prepare and file the CT600 for your UK limited company without missing a deduction or a deadline.
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.
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.
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.
Corporation tax rates 2024-25
| Profit band | Rate |
|---|---|
| Up to £50,000 | 19% (small profits rate) |
| £50,001-£250,000 | 26.5% effective (marginal relief) |
| Over £250,000 | 25% (main rate) |
Marginal relief smooths the transition between £50,000 and £250,000. Associated companies share the bands, so a group of three companies has a small profits limit of just £16,667 each.
Building the tax computation
The computation starts from accounting profit and adjusts:
- Add back depreciation, amortisation, entertaining, fines, 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
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 CT600 guide for line-by-line help. The HMRC Corporation Tax for Company Tax Return guidance is the authoritative source for current forms. See pricing if you want filing built into your bookkeeping rather than bolted on.