What actually gets filed
A Corporation Tax Return — also called a Company Tax Return — is more than one form. When you file Corporation Tax with HMRC, three things are submitted together:
- The CT600 form — the return itself: company details, accounting period, profits, reliefs, and the Corporation Tax calculation. Supplementary pages cover specific situations, such as CT600A for directors’ loans.
- Statutory accounts — your company’s year-end accounts, tagged in iXBRL (a machine-readable format HMRC requires).
- A Corporation Tax computation — the working that turns your accounting profit into taxable profit, also in iXBRL.
This is separate from your Companies House filing. You still send annual accounts to Companies House — usually 9 months after year end — even though HMRC also receives accounts with the CT600. All three HMRC parts are submitted together through filing software: once your figures are ready, you can file your Corporation Tax Return online in one guided workflow.
HMRC’s free filing service closed on 31 March 2026. From 1 April 2026, companies can no longer use HMRC’s online Company Tax Return service. To file a CT600 you now need commercial software, or an accountant who files on your behalf. Paper returns are accepted only in limited cases — a reasonable excuse for not filing online, or filing in Welsh — with form WT1 explaining why.
Before you start: what you need
- Your Corporation Tax UTR — the 10-digit Unique Taxpayer Reference HMRC sent when the company registered for Corporation Tax.
- Company details — registered name and Companies House number.
- The accounting period — normally the same 12 months as your company’s financial year.
- Finalised year-end figures — turnover, expenses, profit, and details of anything that needs adjusting for tax (depreciation, entertainment, capital purchases, directors’ loans).
- Filing software — HMRC-recognised Corporation Tax software that can produce the CT600, computation and iXBRL accounts.
How to file Corporation Tax, step by step
Work out your accounting period
Your Corporation Tax accounting period normally matches your financial year and cannot be longer than 12 months. If your first accounts cover more than 12 months, you will file two returns (see your first return).
Prepare your statutory accounts
Most small companies prepare accounts under FRS 105 (micro-entities) or FRS 102 Section 1A (small companies). These are the accounts that get iXBRL-tagged and attached to the return.
Build the tax computation
Start from accounting profit, add back disallowable costs (depreciation, client entertainment, fines), deduct capital allowances, apply any losses, and arrive at taxable profit. Apply the Corporation Tax rate: 19% small profits rate up to £50,000, 25% main rate above £250,000, with marginal relief in between.
Complete the CT600
Enter the company details, period, income, reliefs and tax due. Add supplementary pages where they apply — most commonly CT600A if a director owed the company money during the period.
Attach the iXBRL files and submit
Your software generates the iXBRL accounts and computation, attaches them to the CT600 and submits the whole return to HMRC online. Keep the submission confirmation with your records.
Pay your Corporation Tax
Payment is due 9 months and 1 day after the accounting period ends — earlier than the filing deadline. Pay electronically using your 17-character Corporation Tax payment reference.
Want the short version? #GoFile turns these steps into one guided workflow: enter your figures, review the calculation, and file your Corporation Tax Return online. Preparing is free — you only pay when you submit.
Worked example: from accounts profit to tax due
Here is a simple computation for a company with a 31 March 2027 year end, a full 12-month accounting period and no associated companies:
| Computation step | Amount |
|---|---|
| Accounting profit (per the accounts) | £68,200 |
| Add back: depreciation | + £4,300 |
| Add back: client entertainment | + £900 |
| Less: capital allowances (AIA on £6,000 of equipment) | − £6,000 |
| Taxable trading profit | £67,400 |
| Corporation Tax at the 25% main rate | £16,850 |
| Less: marginal relief — 3/200 × (£250,000 − £67,400) | − £2,739 |
| Corporation Tax due | £14,111 |
Because taxable profit sits between £50,000 and £250,000, marginal relief applies, giving an effective rate of about 20.9% rather than the full 25%. In this example the tax is payable by 1 January 2028 and the CT600 is due by 31 March 2028. You can test your own figures with our Corporation Tax calculator — depreciation is added back because companies claim capital allowances instead.
Deadlines and penalties
Corporation Tax has two deadlines that catch people out because the payment is due before the return:
| Obligation | Deadline |
|---|---|
| Pay Corporation Tax | 9 months and 1 day after the accounting period ends |
| File the CT600 | 12 months after the accounting period ends |
| File accounts at Companies House | 9 months after the company’s financial year ends (separate filing) |
Late filing penalties
| How late | Penalty |
|---|---|
| 1 day | £100 |
| 3 months | Another £100 |
| 6 months | HMRC estimates your bill and adds 10% of the unpaid tax |
| 12 months | Another 10% of any unpaid tax |
If the return is late three accounting periods in a row, the £100 penalties rise to £500 each. Late payment also accrues interest from the day after the payment deadline.
Filing your first Corporation Tax Return
New companies often trip over one quirk: your first accounts from Companies House usually cover more than 12 months (from incorporation to the end of the month, one year later). A Corporation Tax accounting period cannot exceed 12 months, so for your first year you typically file two CT600s:
- One for the first 12 months of trading.
- One for the remaining days up to your accounts date.
Both returns are due 12 months after the end of the period each covers. After the first year, your accounting period and financial year normally align and you file one return per year. Also make sure the company is registered for Corporation Tax — this happens when you tell HMRC the company is active, normally within 3 months of starting to trade.
Dormant companies and nil returns
Filing a dormant company CT600
If your company is dormant — no trading activity or income — tell HMRC. Once HMRC agrees the company is dormant, it normally will not issue a notice to deliver a return, and no CT600 is needed. But if HMRC has issued a notice to deliver, you must file, even for a dormant period. Filing the return is how you formally show HMRC there was nothing to tax.
Filing a nil CT600
A nil return is a CT600 showing no Corporation Tax due — usually because the company made a loss or had no taxable profit. There is no special “nil return” form: you file the normal CT600 with your accounts and computation, showing zero tax payable. A loss-making company should still file on time; reported losses can be carried forward against future profits.
Fixing mistakes: amending a CT600
You can amend a Company Tax Return within 12 months of the filing deadline. Amendments are made through your filing software by resubmitting the corrected return. If you discover an error after the amendment window, contact HMRC — overpaid tax can often still be reclaimed through an overpayment relief claim.
Software vs accountant: which way to file?
There are two routes to file a Company Tax Return now that HMRC’s own service has closed:
- File it yourself with software. For a small company with straightforward affairs, HMRC-recognised software like #GoFile guides you from year-end figures to a submitted CT600, including the iXBRL attachments, from £49.99/year.
- Use an accountant. Sensible when the company has complex issues — group relief, R&D claims, significant capital transactions — or when you simply want the return handled. Many accountants use #GoFile to file client CT600s.
Either way, the directors remain legally responsible for the return being accurate and on time.
When you should not file your CT600 yourself
Self-filing suits a company whose numbers are simple. Get professional advice before filing your own return if any of these apply for the period:
- R&D tax relief claims — these need the Additional Information Form, supplementary page CT600L and defensible technical justification; HMRC scrutiny of R&D claims is high.
- Groups and associated companies — group relief, transfers between group companies, or working out how associated companies affect your rate thresholds.
- Significant capital transactions — selling property, disposing of the business, share buy-backs or restructuring.
- Overseas income or ownership — foreign branches, non-UK shareholders or transfer pricing questions.
- Audited or larger companies — if the company is beyond the small companies regime, the accounts and tax interact in ways that merit professional review.
- You are already behind — if returns are late or HMRC has opened an enquiry, an accountant will usually save more than they cost.
If none of these apply — one company, UK trading income, straightforward expenses — a guided self-filing is normally realistic.
File your CT600 online with #GoFile
HMRC-recognised Corporation Tax software with iXBRL accounts and computations included. Prepare free, pay when you submit.
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