How to Avoid Errors in Your Company’s CT600 Form and Corporation Tax Filing

Team Nomi

Nov 12, 2025
3 minutes read

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Filing your company’s CT600 form correctly is essential to staying compliant with HMRC and avoiding costly delays or penalties. 

Yet every year, thousands of small businesses make simple mistakes in their Corporation Tax filing. In fact, over half of all small businesses submitted incorrect Corporation Tax returns and around three-quarters of those incorrect returns had an additional Corporation Tax liability of over £1,000. These costly mistakes that can be time-consuming to fix and may even trigger HMRC checks.

The good news? With the right preparation and tools, these errors are completely avoidable. Here’s how to get your CT600 right the first time.

 Double-Check Your Company Details

It sounds obvious, but many submissions are rejected due to incorrect company details. Make sure your CT600 form includes:

• • The correct company registration number (CRN)
    
  • Accurate accounting period dates
    
  • Updated registered office address
    
  • The same figures reported in your Companies House accounts

Even small inconsistencies can cause HMRC to flag your return for review. Always cross-check your data before submitting.

Report All Income and Expenses Accurately

HMRC expects your Corporation Tax filing to reflect your full income and allowable expenses. Omitting revenue or claiming ineligible costs can lead to penalties or extended reviews.

Common mistakes include:

• • Forgetting to include interest received or other non-trading income
    
  • Overstating expenses without valid receipts or evidence
    
  • Mixing personal and business costs

Using reliable accounting software helps capture and categorise these automatically, reducing the chance of human error.

Watch Out for R&D and Capital Allowance Errors

If your business claims Research & Development (R&D) tax relief or capital allowances, accuracy is critical. Many errors happen when figures are entered incorrectly or when qualifying expenditure isn’t properly supported.

Make sure:

• • R&D costs are well-documented and fall within HMRC’s criteria
    
  • Capital allowances are claimed on eligible assets only (like equipment, vehicles, or software)
    
  • Depreciation isn’t mistakenly included as a tax deduction

These sections can make a real difference to your tax bill, so it’s worth reviewing them carefully.

Reconcile Your Accounts Before Filing

Before submitting your CT600 form, check that your accounts are fully reconciled. Bank statements, ledgers and trial balances should all agree with the figures in your Corporation Tax calculations.

If something doesn’t add up, fix it before submission. It’s much easier than trying to amend a filed return later.

Use HMRC-Recognised Software

Manual entry and paper forms increase the risk of mistakes. The easiest way to avoid errors is to use HMRC-approved software for Corporation Tax filing. It helps by:

• • Automatically populating figures from your accounts
  • Running validation checks before submission
  • Keeping digital records for audit trails
  • Avoiding human error

For small businesses, automation takes the stress out of compliance and ensures every return meets HMRC’s requirements.

Accuracy Made Easy

Filing your CT600 form doesn’t have to be a headache. By checking your details, reconciling your accounts and using reliable software, you can avoid the most common pitfalls in Corporation Tax filing and stay fully compliant.

Ready to simplify your next CT600 submission? Nomi’s HMRC-recognised software helps limited companies prepare, check and submit returns accurately, saving time and reducing the risk of error.

Book a free trial or a demo and see how Nomi can help.