The CT600 is the Company Tax Return that every UK limited company must file with HMRC. It calculates how much Corporation Tax your company owes for each accounting period. Even if the company made a loss or had no activity, a CT600 must be filed once HMRC has issued a notice to deliver.
This guide walks through the main sections of the form and the adjustments most small companies need to make.
Before you start
You need the following information to hand:
| Item | Where to find it |
|---|---|
| Company UTR (Unique Taxpayer Reference) | HMRC letter issued after Corporation Tax registration |
| Companies House registration number | Certificate of incorporation |
| Accounting period start and end dates | Your accounting records |
| Finalised profit and loss account | Your accounting software or accountant |
| Balance sheet at period end | Your accounting software or accountant |
| Capital allowances computation | Prepared separately or by your accountant |
| Details of any losses brought forward | Previous year’s CT600 or tax computation |
You also need your statutory accounts in iXBRL format and a Corporation Tax computation, both of which are filed as attachments to the CT600.
Structure of the CT600
The CT600 is divided into sections. Not every section applies to every company. For a straightforward small company, you will mainly use:
| Section | Contents |
|---|---|
| Company information (boxes 1-30) | Basic details: name, UTR, accounting period, type of company |
| Turnover and income (boxes 145-165) | Trading turnover, other income, rental income |
| Trading profits (boxes 155-190) | Adjusted trading profit after tax adjustments |
| Capital allowances (box 195) | Total capital allowances claimed |
| Losses (boxes 220-245) | Trading losses, losses brought forward, losses carried back |
| Tax calculation (boxes 430-470) | Corporation Tax liability, marginal relief, tax due |
| Tax already paid (boxes 510-530) | Payments on account, income tax deducted at source |
| Supplementary pages | R&D, capital gains, group relief, overseas income (only if applicable) |
Key boxes to complete
| Box | What to enter |
|---|---|
| 1-3 | Company name, registration number, UTR |
| 30, 35 | Accounting period start and end dates |
| 145 | Total turnover (gross sales revenue) |
| 155 | Gross trading profits (after adjustments – see below) |
| 160 | Trading losses (instead of box 155, if the company made a loss) |
| 195 | Total capital allowances claimed |
| 220-245 | Losses: current period, carried back, brought forward |
| 430 | Corporation Tax chargeable before reliefs |
| 440 | Marginal relief (if profits are between £50,000 and £250,000) |
| 470 | Net Corporation Tax outstanding |
Make sure the accounting period matches what HMRC expects. If you have changed your year-end date, notify HMRC before filing.
Corporation Tax rates
| Profit level | Rate |
|---|---|
| Up to £50,000 | 19% (small profits rate) |
| £50,001 to £250,000 | Marginal rate |
| Above £250,000 | 25% (main rate) |
If you have associated companies, the £50,000 and £250,000 thresholds are divided by the number of associated companies plus one. Most accounting software calculates the marginal relief automatically.
Adjustments: from accounting profit to taxable profit
The CT600 does not simply apply the tax rate to your accounting profit. You need to make adjustments to arrive at the taxable profit. The most common adjustments for small companies are:
Add back to profit
| Item | Why |
|---|---|
| Depreciation | Replaced by capital allowances for tax purposes |
| Entertaining clients | Not allowable for Corporation Tax |
| Fines and penalties | Not allowable |
| General bad debt provisions | Only specific bad debts are deductible |
| Legal costs relating to capital items | Capital, not revenue |
| Non-business proportion of motor expenses | Private use element |
| Donations to non-qualifying bodies | Only qualifying charitable donations are deductible |
Deduct from profit
| Item | Why |
|---|---|
| Capital allowances | Replaces depreciation |
| Trading losses brought forward | Relieves tax on current profits |
| Qualifying charitable donations | Deducted as charges on income |
| R&D additional deduction | If claiming R&D tax relief |
These adjustments are summarised in the Corporation Tax computation, which is filed as an attachment alongside the CT600.
Required attachments
The CT600 alone is not sufficient. You must also file:
| Attachment | Format | Purpose |
|---|---|---|
| Statutory accounts | iXBRL | Balance sheet, profit and loss, notes |
| Corporation Tax computation | iXBRL or PDF | Shows adjustments from accounting profit to taxable profit |
| Capital allowances computation | Included in CT computation | Details of each allowance claimed |
iXBRL (inline eXtensible Business Reporting Language) is a tagged data format that HMRC’s systems can read automatically. Most accounting and tax software generates iXBRL files. If you are preparing accounts manually, you will need a separate iXBRL tagging tool.
Filing the CT600
All CT600s must be filed electronically. You can either use HMRC’s online service (log in with Government Gateway, enter data manually and upload iXBRL files) or file directly through commercial accounting software, which is faster and reduces data entry errors.
Filing deadline
The CT600 must be filed within 12 months of the end of your accounting period. This is three months after the Corporation Tax payment deadline.
| Accounting period end | Payment deadline | Filing deadline |
|---|---|---|
| 31 March | 1 January following year | 31 March following year |
| 31 December | 1 October following year | 31 December following year |
Penalties for late filing
| Delay | Penalty |
|---|---|
| 1 day | £100 |
| 3 months | Additional £100 |
| 6 months | 10% of unpaid tax (HMRC estimate) |
| 12 months | Additional 10% of unpaid tax |
If your CT600 is late three consecutive times, the £100 penalties increase to £500 each.
Amending and special situations
You can amend a CT600 within 12 months of the filing deadline. After that, only HMRC can make changes through a discovery assessment. File the amendment through the same route you used for the original return.
If your company was dormant or had no trading activity, you must still file a CT600 if HMRC has issued a notice to file. The return will show zero turnover and zero tax, but it must still be filed with the iXBRL accounts attached. If HMRC has confirmed they will not issue notices, you do not need to submit until the company starts trading again.
If your first accounting period exceeds 12 months, you must file two CT600s – one for the first 12 months and one for the remaining period. Income and expenses must be apportioned between them.
After filing
Once HMRC processes your CT600, they may accept it, open an enquiry within 12 months, or issue a discovery assessment later. Keep all supporting records for at least six years after the end of the accounting period.