Understanding Your CT600: Box by Box Explanation

The CT600 is the Corporation Tax return that every UK limited company must file with HMRC. It reports your company's taxable profits, calculates the Corporation Tax due, and must be submitted within 12 months of the end of your accounting period. Whether you are filing for the first time or want to better understand what your accountant submits on your behalf, this guide walks you through the key boxes and sections.

What Is the CT600?

The CT600 is a statutory form that tells HMRC:

• How much profit (or loss) your company made
• How much Corporation Tax is due
• Whether any reliefs or deductions apply
• Whether your company has any associated companies or special circumstances

It must be filed online — HMRC no longer accepts paper CT600 returns. Your company accounts must accompany the return in iXBRL format (Inline eXtensible Business Reporting Language), which is a tagged digital format that HMRC's systems can read automatically.

The Key Boxes Explained

The CT600 contains over 200 boxes, but most companies only need to complete a fraction of them. Here are the ones that matter most:

Company Information (Boxes 1-30)

• Box 1: Company name (as registered with Companies House)
• Box 2: Company registration number
• Box 3: Tax reference (your company's Unique Taxpayer Reference / UTR)
• Box 4: Type of company (e.g., close company, investment company)
• Boxes 30-35: Accounting period start and end dates

Turnover and Income (Boxes 145-165)

• Box 145: Turnover — your total sales revenue for the accounting period. This is one of the most important boxes and must match your profit and loss account
• Box 150: Trading profits — your net trading profit after allowable deductions
• Box 155: Total profits — the sum of all income sources (trading, investment, property, etc.) before deductions
• Box 160: Net chargeable gains — any capital gains made during the period (from selling assets, for example)
• Box 165: Profits before deductions and reliefs — total profits plus chargeable gains

Deductions and Reliefs (Boxes 170-225)

• Box 170: Losses brought forward — trading losses from previous accounting periods that can be offset against current profits
• Box 190: Charitable donations — qualifying donations that reduce taxable profits
• Box 200: Group relief — losses surrendered from other companies in the same group
• Box 225: Profits chargeable to Corporation Tax — this is the figure your tax is calculated on, after all deductions and reliefs

Tax Calculation (Boxes 430-475)

• Box 430: Tax chargeable — the Corporation Tax calculated on your chargeable profits
  • At the small profits rate of 19% if profits are under £50,000
  • At the main rate of 25% if profits are over £250,000
  • At a marginal rate if profits are between £50,000 and £250,000
• Box 440: Corporation Tax chargeable — the final tax amount after marginal relief
• Box 470: Tax payable — the amount you actually need to pay, after deducting any payments already made or tax credits
• Box 475: Tax overpaid — if you have overpaid, this shows the amount due back to you

Capital Allowances (Boxes 490-525)

• Box 490: Annual Investment Allowance — the amount claimed under the AIA (up to £1 million)
• Box 495: Full expensing — 100% first-year allowance on qualifying plant and machinery
• Box 525: Total capital allowances claimed — the total amount of capital allowances deducted from profits

Supplementary Pages

Depending on your company's circumstances, you may need to file additional pages alongside the main CT600:

• CT600A: Loans to participators — required if your company has made loans to directors or shareholders (Section 455 tax)
• CT600B: Controlled foreign companies — for companies with interests in foreign subsidiaries
• CT600C: Group and consortium relief — for companies claiming or surrendering group relief
• CT600D: Insurance — for insurance companies
• CT600E: Charities and CASCs — for charitable companies
• CT600J: Disclosure of tax avoidance schemes — if your company has used a notifiable tax avoidance scheme

Most small companies will only need the main CT600 and possibly CT600A (if the director has a loan account).

The iXBRL Requirement

Since April 2011, all company accounts filed with HMRC as part of a Corporation Tax return must be in iXBRL format. This is not a format you can create in Word or Excel — it requires specialist software or a service that produces iXBRL-tagged files.

Key points about iXBRL:

• Each financial figure in your accounts is tagged with a standardised taxonomy label
• HMRC's systems use these tags to automatically extract and validate data
• Incorrect or missing tags can cause your filing to be rejected
• The taxonomy used is the UK GAAP taxonomy (for FRS 102/105 accounts)

This requirement alone is one of the main reasons many directors use professional services or tools like TaxDocs rather than attempting to file manually.

Online Filing

Corporation Tax returns must be filed online using either:

• HMRC's own filing service (limited functionality, no iXBRL support for accounts)
• Commercial software that is recognised by HMRC
• An agent (accountant or tax adviser) filing on your behalf

You will need your company's Government Gateway credentials and your UTR number to file.

Common Mistakes to Avoid

1. Entering turnover incorrectly — Box 145 must match your accounts. Discrepancies trigger automated queries
2. Forgetting to include all income sources — investment income, rental income, and bank interest must all be reported
3. Not claiming all available reliefs — capital allowances, R&D credits, and marginal relief are frequently missed
4. Director's loan account errors — if the director owes the company money at the year-end, Section 455 tax may be due
5. Missing the filing deadline — automatic penalties start at £100 and escalate. Interest is charged on late payment
6. Incorrect accounting period dates — must match exactly with your previous return and Companies House records

How TaxDocs Generates Your CT600 Automatically

Completing a CT600 correctly requires understanding tax law, accounting standards, and HMRC's filing requirements. TaxDocs eliminates this complexity:

1. Upload your bank statements and company information
2. AI categorises all transactions and calculates your taxable profits
3. The system generates your complete CT600 with all relevant boxes populated
4. iXBRL-tagged accounts are produced alongside the return
5. Review and file — everything is ready for submission to HMRC

No need to understand what goes in Box 155 or worry about iXBRL tagging. TaxDocs handles it all, starting from just £29 per filing.

Key Takeaways

• The CT600 is mandatory for every UK limited company
• Focus on the key boxes: 145 (turnover), 155 (total profits), 225 (chargeable profits), and 440 (tax chargeable)
• iXBRL accounts must accompany your return — you cannot file without them
• File online within 12 months of your accounting period end
• Avoid common mistakes that trigger HMRC queries and penalties
• Consider using TaxDocs to automate the entire process accurately

This article is for informational purposes only and does not constitute tax advice.