For every UK company, filing the CT600 Corporation Tax return with HM Revenue & Customs (HMRC) is one of the most critical compliance obligations. It is not optional. Whether you are running a high-growth startup or managing an established limited company, CT600 is the mechanism that ensures your Corporation Tax liability is calculated, reported, and paid correctly.
But for many business owners and finance leaders, the process is confusing. The requirements are highly specific, the deadlines are strict, and the penalties for errors or late submissions can be costly. This guide takes you through everything you need to know: what the CT600 is, when and how to file it, the risks of non-compliance, and strategies to simplify the process.
What Is the CT600 Corporation Tax Return?
The CT600 is the official Corporation Tax return form required by HMRC. It sets out:
- The company’s income, expenses, and taxable profits for a given accounting period
- The Corporation Tax owed, based on current HMRC rates
- Any tax reliefs, allowances, or credits claimed (such as R&D tax credits or capital allowances)
- Losses carried forward or back
- Other adjustments affecting the final Corporation Tax position
All companies registered with Companies House may be required to submit a CT600, even those with no profits or dormant activity. If HMRC issues a “Notice to Deliver a Company Tax Return,” you must file, regardless of trading status.
Who Needs to File a CT600?
The CT600 applies to:
- Private limited companies (Ltd)
- Public limited companies (PLC)
- Non-UK companies with taxable income in the UK
- Clubs, associations, or charities with chargeable gains or taxable activities
Dormant companies may be exempt from Corporation Tax, but if HMRC sends a notice to deliver, a CT600 is still required. This is a common compliance trap, many directors assume inactivity removes the obligation, but HMRC expects a return if requested.
CT600 Filing Deadlines
The deadlines are twofold and often confused:
- Payment deadline: Corporation Tax must be paid within 9 months and 1 day after the end of the accounting period.
- Filing deadline: The CT600 return must be submitted within 12 months of the end of the accounting period.
Example: If your accounting year ends on 31 December 2024:
- Payment must be made by 1 October 2025.
- CT600 return must be filed by 31 December 2025.
The payment deadline arrives before the filing deadline, creating a timing challenge for many businesses. Accurate tax calculations must be prepared early.
Penalties for Late CT600 Filing
HMRC imposes penalties automatically for late filings:
- 1 day late: £100 fine
- 3 months late: Additional £100 fine
- 6 months late: HMRC estimates your tax liability and adds a 10% penalty
- 12 months late: Another 10% of unpaid tax
Repeat late filers face higher penalties. Interest is also charged on late payments.
The real risk extends beyond penalties. Late or incorrect filings undermine credibility with investors, auditors, and regulators. For growing businesses, this can create barriers during funding rounds or due diligence.
Step-by-Step Guide to Filing the CT600
Step 1: Prepare Statutory Accounts
You must prepare full statutory accounts, including:
- Profit and loss statement
- Balance sheet
- Notes to the accounts
- Director’s report
These are the foundation of the CT600. The figures must reconcile with tax computations.
Step 2: Adjust for Tax Purposes
Accounting profits and taxable profits differ. You must adjust for:
- Disallowable expenses (e.g., client entertainment)
- Capital allowances on qualifying assets
- Loss reliefs or group relief
- Tax credits (such as R&D)
This ensures the taxable profit figure is accurate.
Step 3: Register for HMRC Online Services
If not already registered, sign up for HMRC Corporation Tax online services using your company’s Unique Taxpayer Reference (UTR). This enables electronic filing, which is mandatory.
Step 4: Choose a Filing Method
There are two primary options:
- HMRC online filing service: Suitable for simple companies.
- Commercial HMRC-approved software: Recommended for complex filings, group companies, or businesses claiming reliefs.
Step 5: Complete the CT600 Form
Sections include:
- Company details and UTR
- Accounting period dates
- Taxable profits or losses
- Reliefs and allowances
- Final tax calculation
Accuracy is critical. Misreporting, even unintentionally, can trigger HMRC enquiries.
Step 6: Attach iXBRL Accounts and Computations
Accounts and computations must be tagged in iXBRL (Inline eXtensible Business Reporting Language) format. HMRC will reject submissions without correct tagging.
Step 7: Submit and Pay
- Submit electronically via HMRC or approved software.
- Pay Corporation Tax by bank transfer, BACS, CHAPS, or Direct Debit.
- Retain acknowledgment and receipts for compliance records.
Common Mistakes in CT600 Filing
Even diligent companies fall into common traps:
- Filing late – Confusion between filing and payment deadlines.
- Incorrect iXBRL tagging – Leading to rejected returns.
- Ignoring HMRC notices – Failing to file when required, even with no profit.
- Overlooking reliefs – Missing out on R&D credits, capital allowances, or group relief.
- Using accounting profit instead of adjusted profit – Resulting in incorrect tax liabilities.
- Disorganized records – Leading to errors or HMRC enquiries.
Each of these mistakes increases compliance risk and, in some cases, financial loss.
Example: CT600 for a Profitable SME
Suppose your company reports an accounting profit of £500,000. Adjustments include:
- Disallowable expenses: £15,000
- Capital allowances: £25,000
- R&D tax credit: £50,000
Taxable profit = £500,000 + £15,000 – £25,000 – £50,000 = £440,000.
At a Corporation Tax rate of 25%, liability = £110,000.
This calculation is disclosed in the CT600, with supporting accounts and computations attached.
Software and Tools for Filing CT600
- HMRC’s online filing service – Simple, free, but limited.
- Integrated compliance platforms – Commenda and similar platforms combine CT600 filing with entity management, deadlines, and tax oversight.
Compliance Tips for CT600 Filings
- Start preparing early, ideally three months before the deadline.
- Use professional software for accuracy and efficiency.
- Double-check reliefs and allowances to avoid overpayment.
- Keep internal records aligned with filings.
- Consider professional review for complex structures or cross-border entities.
Why Businesses Outsource CT600 Filings
For founders and finance leaders, the challenges are clear:
- The technical complexity of tax adjustments
- The administrative burden of iXBRL tagging
- The risk of errors leading to penalties or HMRC scrutiny
- The need to reconcile filings with statutory accounts
- Managing multiple filings across entities and jurisdictions
Outsourcing ensures compliance, accuracy, and efficiency, allowing businesses to focus on growth instead of tax filings.
How Commenda Simplifies CT600 Filing
Commenda is an all-in-one compliance and tax platform trusted by cross-border companies. For CT600 and Corporation Tax compliance, Commenda provides:
- Automated compliance calendar – Tracks filing and payment deadlines.
- Seamless digital filing – HMRC-approved submissions with iXBRL tagging built in.
- Expert support – Tax professionals review and optimize your filings.
- Centralized records – Store accounts, computations, and filings in one secure hub.
- Global compliance – Manage UK Corporation Tax alongside international obligations.
With Commenda, companies eliminate filing errors, meet deadlines with confidence, and ensure reliefs and allowances are properly claimed.
Book a demo with Commenda to simplify CT600 filings and protect your business from compliance risks.
FAQs: CT600 Corporation Tax Returns
1. Do all companies need to file a CT600?
Yes, if HMRC issues a notice. Even dormant or loss-making companies must file.
2. Can I file CT600 on paper?
No. HMRC requires electronic filing with iXBRL-tagged accounts and computations.
3. What happens if I miss the deadline?
Automatic penalties apply, starting at £100 and escalating with time.
4. Can I amend a CT600 after filing?
Yes, amendments can be made within 12 months of the filing deadline.
5. Is the payment deadline the same as the filing deadline?
No. Payment is due nine months and one day after the accounting period ends; filing is due three months later.
6. Can I file CT600 myself?
Yes, but many businesses prefer professional or outsourced support due to complexity.
