Corporation Tax is one of the most important and often misunderstood aspects of running a limited company in the UK. At the heart of it is the CT600, a form that HMRC requires every active company to submit annually. While it might seem like just another piece of compliance, getting your CT600 wrong (or filing it late) can lead to overpaid tax, penalties, or unwanted attention from HMRC.

In this guide, we’ll walk you through what the CT600 is, who needs to file it, what goes into it, and — most importantly — how to avoid common mistakes. We’ll also explain how Pro AccX can help you stay compliant, reduce tax liabilities, and take the stress out of Corporation Tax season entirely.

What Is a CT600 Form?

The CT600 is a formal Corporation Tax return submitted to HMRC. Every UK limited company that is “active” for tax purposes must file one return each year. Even if your company doesn’t make a profit, or if your business made only a small amount of income, the CT600 is still legally required.

This form details your total income, trading profits, allowable expenses, and the calculation of the Corporation Tax due. It acts as the central document that confirms how much your business owes HMRC, or if you’re due a refund.

For many small business owners, the CT600 can be confusing because it often includes technical language, statutory formats, and tax adjustments that don’t clearly align with day-to-day bookkeeping. That’s where professional help makes all the difference. At Pro AccX, we break down the CT600 into clear, understandable steps, ensuring it’s filed correctly and in a way that maximises any reliefs you’re entitled to.

Who Needs to File a CT600?

In short, if you’re running a limited company that is actively trading or earning interest or investment income, then you’re expected to submit a CT600 every year.

This includes:

• UK-registered limited companies
• Foreign companies with a UK branch or office
• Nonprofits or charities with taxable income
• Clubs and associations operating as incorporated entities

Even dormant companies, those not currently trading, may need to file a “nil return” or inform HMRC of their dormant status. It’s not enough to assume you’re exempt; HMRC needs to be notified officially, and failure to do so could still trigger penalties.

If you’re unsure whether your company needs to file, or if you’re trading part-time or irregularly, Pro AccX can review your business status and ensure you stay on the right side of HMRC.

Filing Deadlines and Payment Due Dates

Understanding the timeline is crucial, as late submissions can quickly become expensive. The two key deadlines are:

• Pay Corporation Tax: Within 9 months and 1 day after your accounting year ends
• File your CT600 Return: Within 12 months of your year-end

For example, if your company’s year ends on 31st December 2024:

• Corporation Tax must be paid by 1st October 2025
• The CT600 form must be submitted by 31st December 2025

Many small business owners mistakenly believe they only need to submit accounts to Companies House, but HMRC’s CT600 filing is entirely separate and must be done in iXBRL format. This format involves “tagging” financial data so that HMRC systems can read it digitally. It’s a technical step that most small business owners aren’t equipped to handle, and is often where DIY returns go wrong.

At Pro AccX, we use specialist software and filing systems that are fully HMRC-compliant and take care of these formatting requirements as part of our Corporation Tax service. You won’t need to worry about digital submissions, tagging, or formatting errors; we’ll handle it all.

What Goes Into the CT600?

The CT600 isn’t just a copy of your company accounts. It’s a tax-specific document that includes adjustments, disclosures, and calculations that are often unfamiliar to small business owners.

Here’s what the CT600 typically includes:

• Total company income (turnover, interest, investments)
• Trading and non-trading profits
• Allowable expenses and adjustments
• Capital allowances for equipment or vehicles
• Any R&D claims, tax reliefs, or group reliefs
• Dividends paid to shareholders
• Corporation Tax calculation
• Director’s loans or benefits in kind (where relevant)

The form may also include supplementary pages depending on your activity, such as those for chargeable gains, charitable donations, or property income. The complexity of the CT600 increases with the size and scope of the business.

Our CT600 Service at Pro AccX ensures that every detail, from expenses and allowances to tax calculations and disclosures, is handled with accuracy and compliance. We also review all available tax reliefs, ensuring you don’t overpay or miss opportunities for tax savings.

Common Mistakes Business Owners Make with CT600

Filing a CT600 isn’t just about ticking boxes. Even small errors can result in HMRC fines or incorrect tax bills. Here are some of the most common mistakes we help clients avoid:

1. Filing Late
It happens all the time: a client forgets their deadline or assumes their accountant is handling it. But late CT600 filings lead to an immediate £100 penalty. After 3 months, it becomes £200. After 6 months, HMRC may estimate your tax, often inflating what you owe.

We use automated reminders and deadline tracking to ensure our clients never miss a date again.

2. Not Claiming Allowances
Businesses frequently miss out on capital allowances — like those for laptops, equipment, or vehicles — which could significantly reduce their Corporation Tax bill.

We help you identify and claim every allowance you’re entitled to, keeping more money in your business.

3. Incorrect iXBRL Tagging
Even if your figures are accurate, submitting the CT600 in the wrong format means HMRC will reject your return. Many business owners attempt to file manually and fall at this hurdle.

Pro AccX uses HMRC-approved software that tags and files everything in the required digital format, ensuring smooth submission.

4. Misreporting or Mixing Personal Expenses
Claiming personal expenses as business costs is a major red flag. Whether it’s a home office claim gone wrong or unclear travel expenses, misreporting like this can trigger an investigation.

We double-check every entry and advise clients on what’s truly allowable under HMRC rules.

The Advantages of Filing Correctly and Professionally

When your CT600 is filed correctly, the benefits go beyond avoiding fines. Accurate Corporation Tax returns can:

• Maximise your profit by reducing overpaid tax
• Build a positive track record with HMRC
• Help you qualify for loans, grants, or investments
• Show transparency to shareholders or partners
• Reduce stress and admin during busy year-ends

Most importantly, it gives you peace of mind knowing your business is compliant and tax-efficient. That’s exactly what Pro AccX aims to deliver for every client.

Why Work With Pro AccX?

Filing your Corporation Tax return is too important to leave to chance, and too complex for generic accounting software to handle correctly. That’s where we come in.

At Pro AccX, we specialise in helping small and medium-sized UK businesses manage their tax responsibilities with confidence. Our team handles:

✔ CT600 preparation and iXBRL formatting
✔ Accurate tax calculations and allowance optimisation
✔ Advice on reducing future Corporation Tax bills
✔ Submission tracking and deadline reminders
✔ Full support across bookkeeping and statutory accounts

We understand the unique pressures of running a small business, especially when finances are tight or time is limited. Our service is tailored, affordable, and built around the needs of growing UK businesses.

📞 Ready to File with Confidence?

Don’t risk errors or penalties. Let us take care of your CT600 and ensure your Corporation Tax is filed correctly, on time, and with your best interests in mind.

👉 Call us today or request a quote via our Corporation Tax Returns page
📧 Or drop us a message to get started. Our friendly team is here to help.