If you run a Limited Company in the UK, there’s one form you can’t ignore: the CT600. It’s how your company tells HMRC what it earned, what it spent, and how much Corporation Tax is due. Even if your business hasn’t traded, you will likely still need to submit one.

Think of it as a Self Assessment tax return, but for your company instead of you personally. It’s mandatory, important, and at times a little technical, but this guide will break it down in plain English.

What is a CT600 form?

A CT600 form is the official Corporation Tax return for UK companies. Every year, you use it to report your company’s financial activity for a specific accounting period and calculate how much Corporation Tax you owe. Filing it correctly keeps your company compliant and helps avoid penalties.

What’s inside the CT600?

When completing a CT600 form, you’ll usually include:

1. Your company’s income, including turnover and other earnings.
2. Business expenses and costs.
3. Profits or losses.
4. Tax reliefs and allowances.
5. The final Corporation Tax calculation.

Once submitted, HMRC uses this information to confirm your tax bill and ensure your company stays compliant.

Who needs it?

In short, almost every UK company needs to do one. If your business is registered with Companies House, you will usually need to submit a CT600 each year. This includes:

• Limited Companies.
• Public Limited Companies (PLCs).
• Community interest companies.
• Some clubs, societies, and associations.
• Non-UK companies with UK operations.

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What about dormant companies?

Even if your company isn’t trading, you may still need to submit a CT600. HMRC defines a dormant company slightly differently from Companies House, so it’s always worth checking. In many cases, a “nil” return is enough, but you still need to file it.

If you have formally told HMRC that your company is dormant for Corporation Tax and they have accepted it, you normally will not need to file a CT600 each year. HMRC will not require a return unless they issue a formal Notice to Deliver CT603 for a specific accounting period. If you do receive one, even for a dormant company, you must file a return, usually a nil return, to remain compliant.

It is worth keeping your HMRC dormancy record up to date to avoid unnecessary filings.

Why does the form matter?

It’s easy to think of a CT600 form as just admin, but it’s more than that. Filing correctly is a legal requirement for company directors, keeping you compliant and avoiding penalties.

It also determines your tax bill, so you don’t underpay or overpay, both of which can cost your business extra. You can also claim reliefs that reduce your Corporation Tax bill, including capital allowances, R&D tax relief, and loss relief. Filing properly protects your company and can even save money if you claim everything you’re entitled to.
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CT600 deadlines, penalties and why they matter

The CT600 isn’t just paperwork. Filing it correctly is a legal requirement for company directors and protects your business. It shows HMRC what your company earned, what it spent, and how much Corporation Tax is due. Getting it right helps you avoid mistakes, claim all the reliefs you’re entitled to, and stay fully compliant. 

Your CT600 must be filed within 12 months of the end of your company’s accounting period, and Corporation Tax itself is usually due 9 months and 1 day after the end of that period. Even dormant or loss-making companies need to make these deadlines.

Failing to submit on time or making errors can trigger costs. HMRC can issue a £100 penalty for a late return, and repeated delays increase the charges. Underpaying Corporation Tax leads to interest and extra costs, while overpaying ties up money that could be used elsewhere in your business. 

Filing accurately and on time avoids these headaches, keeps your company in good standing, and makes managing your business simpler and more predictable. 
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What does the form include?

The CT600 isn’t just a simple form you fill in once and forget. It’s part of a Company tax return, which usually includes a number of things.

1. The CT600 form itself

The main document that summarises your company’s tax position and shows HMRC how much Corporation Tax you owe.

2. Company accounts

Your official financial statements for the year, prepared under company law, showing income, expenses, and overall profit or loss.

3. Tax computations

A detailed breakdown of how you moved from your accounting profit to your taxable profit, including adjustments and allowances.

4. Supporting documents

If you’re claiming reliefs or allowances, or if you have more complex tax situations, you may need to provide supporting evidence. 

Think of the CT600 form as the headline, and the accounts and computations as the supporting evidence behind it. 
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CT600 vs annual accounts: what’s the difference?

Many business owners mix these up. Here’s a simple way to see the difference:

In short, annual accounts tell the story of your company, while the CT600 tells HMRC how much tax that story generates. Both are mandatory, but they serve completely different purposes.
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How to submit the CT600 form

Most CT600 forms must now be submitted online, either through HMRC’s online services or using HMRC-approved accounting software like Crunch. Paper submissions are rare and only allowed in limited circumstances.

To file successfully, you’ll need the completed CT600 form showing your tax position, your company accounts in iXBRL format, and your tax computations in iXBRL. Many businesses use accounting software or an accountant to make this easier, reduce errors, and save time.

Do you need an accountant?

You don’t have to, but many directors choose professional help, and for good reason. Handling it yourself might work if your company is very simple with minimal transactions.

Professional help from a Limited Company accountant like Crunch is wise if your finances are more complex, you have employees or VAT, or you want to make the most of your tax reliefs. Even small errors can lead to penalties, so many companies find support worthwhile. Services like our Tax Optimiser can also help Limited Companies save money on their tax bill.

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Understanding the importance of the CT600

The CT600 form is central to running a UK Limited Company. It shows HMRC what your business earned, what it spent, and how much tax is due. Filing properly keeps your company on the right side of the law and protects you as a director.

Even if your company is dormant or hasn’t made a profit, the CT600 still matters. Filing late or making mistakes can be stressful and costly. Think of the form as your company’s official tax story. Handle it properly, and it becomes a simple, routine part of running your business.

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