How to file company tax in the UK (step-by-step).

Most directors mean corporation tax when they say "company tax". Here's the plain-English version of what you normally need to do — deadlines, preparing accounts, preparing the CT600, filing, and paying.

Enter and file your prepared figures

SimpleCompanyTax#Simple CompanyTax uses the following journey for eligible micro-entity companies. You choose the accounting and tax treatment. For clubs, charities and CICs, choose the guide for your organisation.

1. Add the company. Select Add company, use the Companies House option and select your company. Open Start new filing, or continue the existing draft you intend to finish.
2. Set up the filing. In step 1, choose the accounting period, then check the dormancy and special-regime answers. Choose What are you filing? and What the company does. If tax dates differ, follow the separate-date steps.
3. Enter Financial data. Use Profit & loss, Balance sheetand Tax adjustments. Enter prepared category totals, not a single surplus figure. Use the expense map or property map for the exact rows. Supply prior-year comparatives where requested.
4. Complete any two-return allocations. Enter full-period accounts figures once. Use the dated panels below the applicable claims and event amounts, then check that each split adds to its total.
5. Review & Submit. Check the summary and choose Review Filing. Prepare your draft preview, enter the signatory and approval date, and read the accounts, computation and CT600. If there are two returns, read both. Correct entries and regenerate the preview after changes.
6. Submit the selected filing. Complete the subscription step when requested, enter credentials for the chosen authorities and read the declaration before approving. A preview is not a submission.
7. Check Your filing record. Confirm acceptance separately for each authority and each CT600. Use View or Download to keep the documents and receipts.

What is “company tax”?

In the UK, limited companies pay corporation tax on their taxable profits. This is different from income tax (which applies to individuals and sole traders). Your company files a tax return — commonly called a CT600 — to HMRC after each accounting period. If you're not sure what a CT600 is, read: What is a CT600?

1) Know your deadlines

• Your company has an accounting period (usually 12 months).
• You typically file accounts to Companies House and a company tax return to HMRC.
• Companies House accounts are generally due 9 months after the accounting period end.
• The corporation tax return (CT600) is generally due 12 months after the accounting period end.
• Corporation tax payment is typically due 9 months and 1 day after the period end.

Missing deadlines results in penalties and interest. File early, don't leave it to the last week.

2) Prepare your accounts

Your accounts summarise the company's financial activity during the accounting period. Micro-entities often use micro-entity style accounts (FRS 105), which means reduced disclosures and simpler formatting. If you're unsure whether you qualify, read: What is a micro-entity?. For most single-director companies, the accounts include:

• A balance sheet
• A profit and loss summary (or income and expenditure statement)
• Notes (minimal for micro-entities)

3) Prepare your company tax return

HMRC's company tax return is the CT600. It captures:

• Your accounting period dates
• Your taxable profit (which may differ from your accounting profit after adjustments)
• Capital allowances
• Any reliefs claimed
• Corporation tax due

Plain-English explanation: What is a CT600?

4) Submit online

Filing is done digitally. Accounts go to Companies House and the CT600 goes to HMRC. Submitting involves structured formats: iXBRL for accounts (XML-based, human-readable and machine-readable), XML for the CT600 return, and an IRmark — a digital fingerprint HMRC uses to verify the submission hasn't been tampered with. You don't need to understand these formats in detail — a good filing tool handles them for you.

File company accounts & CT600 online →

5) Pay any corporation tax due

If your company has taxable profits, you will owe corporation tax. Payment is typically made through HMRC's online payment system. The due date is usually 9 months and 1 day after the accounting period end. Filing the return and paying the tax are separate steps — filing does not automatically trigger payment.

Already paid some or all of the bill? Follow our guide to filing a CT600 after paying Corporation Tax, including box 595 and the remaining balance.

Common mistakes

• Filing late — penalties apply even if no tax is due.
• Confusing profit with taxable profit — tax adjustments (like depreciation vs capital allowances) change the number.
• Not filing because the company made no profit — you may still need to file. Read: Do I need to file company tax?
• Assuming a dormant company has no obligations — dormant companies may still need to file. Read: Dormant company tax return

Want to avoid accountant-level complexity?

If your company is simple, self-filing is realistic for most directors. You can file company accounts and CT600 online in one place — HMRC and Companies House, from £10 +VAT/year. Still unsure? Can I file company tax myself?