Filing a Company Tax Return (CT600) requires submitting your tax return form, annual statutory accounts, and a tax computation to HMRC. The HMRC free joint filing portal (CATO) officially closed on 31 March 2026. Now, all limited companies must use HMRC-approved commercial software or an accountant to file their CT600.
This guide is going to walk you through exactly how to file CT600 company tax return UK smoothly. You’ll get to know:
- What information do I need to file a CT600?
- How to file CT600 for dormant company?
- Can I file my own CT600?
- And much more…
Let’s get into it!
What Is a CT600 Company Tax Return?
Before diving into how to file CT600 company tax return UK, we need to cover the basic groundwork. A CT600 is the official document that your limited company uses to report its income, expenses, allowances, and final Corporation Tax calculations directly to HMRC.
When we talk about filing a CT600, we mean a full package that includes three things:
- The CT600 form
- Your statutory accounts
- Your tax computations
All of this has to be submitted together. And they must be submitted in a digital format called iXBRL.
Even if your company made a loss, or if you think you owe nothing, HMRC still expects you to file company tax return CT600.
If you are handling this yourself, figuring out how to file CT600 company tax return UK properly ensures you do not get hit with automatic penalties.
Who Needs to File a CT600?
Most UK limited companies must file a CT600 company tax return with HMRC every accounting period. That includes companies that traded, made a loss, or had only a small amount of income.
Here is the simple rule to keep in mind:
- Trading companies usually need to file.
- Companies with investment income usually need to file.
- Dormant companies may still need to file a final return, especially if HMRC has already sent a notice to deliver.
If you are unsure if you are on HMRC’s radar, you can use the official GOV.UK Company Tax Returns Overview to check your status.
What Information Do I Need to File a CT600?
Before we get into how to file CT600 company tax return UK, gather the following:
- Company name as registered with Companies House
- Company Registration Number (CRN)
- Corporation Tax Unique Taxpayer Reference (UTR) from HMRC
- Start and end dates of your accounting period
- Statutory accounts (profit and loss account, balance sheet)
- Details of all income, including trading income and any investment income
- A breakdown of allowable expenses
- Details of any reliefs or allowances you are claiming (capital allowances, R&D credits, etc.)
- Information on associated companies if applicable
- Your Government Gateway login linked to your company
If your accounts are not yet finalised, you cannot file company tax return CT600. The CT600 and your accounts need to be consistent with each other. This is because HMRC cross-checks the figures. It makes the whole process of how to file CT600 company tax return UK much easier if everything matches first.
How to File CT600 Company Tax Return UK: Step by Step
In order to file company tax return CT600, you must register your company for Corporation Tax with HMRC within three months of starting to trade.
If you are wondering exactly how to file CT600 company tax return in the UK, this CT600 step by step breakdown walks you through the essential phases to keep your business fully compliant.
Step 1: Know Your Exact Accounting Period
Your Corporation Tax accounting period usually matches your company’s financial year. But that is not always the case. If your accounting period is longer than 12 months (common in your company’s first year), HMRC will split it into two separate CT600 periods.
This is because a single return cannot cover more than 12 months. So you need to confirm the exact start and end dates before you begin preparing the return. It is a critical first step when sorting out how to file CT600 company tax return UK.
Step 2: Work Out Your True Taxable Profit
Your company accounting profit is rarely the exact figure you pay tax on. When filing a corporation tax return with HMRC, you have to adjust that number to find your taxable profit.
- Add Back Disallowable Expenses: Expenses like business entertainment, corporate gifts, and asset depreciation cannot be used to lower your tax bill. So you must add these back to your net profit.
- Claim Capital Allowances: Deduct the cost of eligible business equipment, machinery, or vehicles. You can deduct these costs using allowances like the Annual Investment Allowance (AIA) to legally reduce your final tax bill.
Step 3: Fill in the CT600 Form Sections
Once your figures sit cleanly in your software, you will work through the specific boxes of the CT600 step by step layout.
First, you fill out the company details section. This includes your company name, registration number, and also the specific dates of your accounting period.
Next, you move to the core financial boxes. This section can get confusing when learning how to file CT600 company tax return UK. But the software usually guides you through it.
Step 4: Attaching the Supplementary Files
HMRC requires you to attach your full iXBRL financial accounts. And also your detailed tax computations to the submission.
If you try to upload a basic PDF or a spreadsheet of your accounts, the HMRC gateway will automatically reject it. Your commercial software must package these separate documents into a single, fully-tagged digital bundle before you can successfully file company tax return CT600 data.
If you’re unsure, check this CT600 form guide by HMRC.
Step 5: Submitting to HMRC and Keeping Records
Review everything one final time before you file company tax return CT600. Check that your profit and loss numbers match your balance sheet perfectly. Verify that your UTR is correct.
When you feel confident, use your commercial software to push the return through to HMRC via the secure gateway. This is the final milestone when filing corporation tax return HMRC submissions, and it completes your process of how to file CT600 company tax return UK.
The system will generate an electronic acknowledgement receipt. Print this out or save it to a secure drive. You are legally required to keep your business tax records for at least six years from the end of the accounting period.
Can I File My Own Corporation Tax Return?
Once you know how to file CT600 company tax return UK, you might wonder: Can I file my own CT600? The simple answer is yes, you can. There is no legal requirement to hire an accountant. If you have an excellent handle on bookkeeping and financial software, you can manage the submission yourself.
The real question you need to ask yourself is whether you actually should.
The process has more moving parts than it might first appear. You now have to buy commercial software yourself. More importantly, you take on all the legal liability if you misinterpret a tax rule or accidentally claim an incorrect expense. Or if you miss a filing deadline.
This is why many owners prefer to let professionals handle how to file CT600 company tax return UK tasks. Passing the burden to an experienced accounting team removes the stress. It also ensures you take advantage of every legitimate tax relief available.
How Long Does it Take to File a CT600?
For a small company with organised records, filing may only take a few hours. If bookkeeping needs updating or accounts have not yet been prepared, the process can take considerably longer.
Complex companies with multiple income streams, investments, or tax relief claims will usually require more time. Know that preparation often takes longer than the actual submission itself. Learning how to file CT600 company tax return UK takes patience. Especially when tracking down old receipts.
How to File CT600 for Dormant Company?
If you set up a limited company but haven’t started trading yet, or if you paused all business operations, your company is considered dormant by HMRC.
You only need to file a CT600 if HMRC specifically sent you a “Notice to Deliver a Company Tax Return” (Form CT603). If they have not issued this notice, you can skip filing the CT600 entirely by simply notifying HMRC online that your company is dormant.
To file a CT600 for a dormant company, you only need to submit a ‘nil’ or blank return if HMRC has explicitly sent you a “Notice to Deliver a Company Tax Return” (CT603). Otherwise, once you notify HMRC of dormancy, you won’t need to file.
Learning how to file CT600 company tax return UK for a dormant business is mostly about keeping HMRC informed.
The Bottom Line
Understanding how to file CT600 company tax return UK is an important part of running a limited company.
You need to know your accounting period dates. You also need to get your accounts and tax computation ready before the payment deadline. Use recognised software to file your CT600 in iXBRL format. Check for reliefs and allowances before finalising your figures.
How Accotax Can Help
If you are still unsure of how to file CT600 company tax return UK, visit Accotax. We offer a range of packages designed to fit your unique needs!
Reach out, get an instant quote, and let us help you stay compliant!
Disclaimer: All the information provided in this article on “How to File CT600 Company Tax Return UK (Step by Step)” including all the texts and graphics, is general in nature. It does not intend to disregard any of the professional advice.
