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

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.

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/year. Still unsure? Can I file company tax myself?