CT600 guide

What the CT600 is

The CT600 is the company tax return UK limited companies file with HMRC each year. It reports the company's taxable profit, calculates the Corporation Tax owed, and is filed alongside the company's statutory accounts and tax computations.

It must be filed online via HMRC's portal or compatible accounting software, and it must include iXBRL-tagged accounts and computations.

What you need before you start

• Statutory accounts for the period
• Detailed tax computations adjusting accounting profit to taxable profit
• Capital allowances workings (Annual Investment Allowance, full expensing, writing-down allowances)
• Records of any losses brought forward or group relief
• Details of dividends paid in the period
• Details of associated companies (affects tax thresholds)
• Your company UTR and Government Gateway login

Key sections of the CT600

• Boxes 1-4: Company details and accounting period
• Boxes 145-160: Trading and other profits
• Boxes 165-205: Loan relationships, intangible fixed assets, capital gains
• Boxes 285-320: Reliefs and deductions (loss relief, group relief, charitable donations)
• Boxes 325-440: Corporation Tax calculation, including small profits and marginal relief
• Supplementary pages: CT600A (close company loans), CT600B (controlled foreign companies), and others as relevant

Common mistakes

• Forgetting to claim available capital allowances (Annual Investment Allowance, full expensing)
• Misallocating capital vs revenue costs
• Missing R&D tax relief if eligible
• Not reporting director's loan accounts (CT600A)
• Filing accounts with Companies House but not the CT600 with HMRC — they're separate filings
• Wrong number of associated companies — divides the £50,000 / £250,000 thresholds

After filing

HMRC has 12 months from the date of filing to open an enquiry into the return. Keep all records — accounts, computations, supporting evidence — for at least six years from the end of the accounting period.