Filing a Company Tax Return is one of the most critical obligations that comes with running a UK limited company. Yet many directors treat it as something their accountant handles in the background, without fully understanding who is legally on the hook if something goes wrong.
The short answer: you are. As the director, the responsibility for making sure your company tax return is filed accurately and on time sits with you, regardless of whether you prepare the return yourself or delegate the work to a professional.
This guide covers what the CT600 involves, which deadlines matter, what penalties HMRC applies, and where directors most commonly get caught out.
What Is a Company Tax Return?
A Company Tax Return, formally known as a CT600, is the document your limited company uses to report its taxable profits and calculate its Corporation Tax liability to HMRC. Every UK limited company must file one for each accounting period, whether the company has made a profit, broken even, or recorded a loss. Even dormant companies may need to file if HMRC has issued a notice to deliver a return.
The CT600 itself is a summary. It brings together your company’s statutory accounts, a Corporation Tax computation, and any claims or elections your company is making for that period.
A common misunderstanding among directors is that having a set of accounts prepared is the same as having a tax return filed. It is not. The accounts show your accounting profit, the computation adjusts that figure for tax purposes, and the CT600 ties everything together for submission.
All CT600 filings must be submitted electronically using HMRC’s approved process, with accounts and computations in iXBRL format. Paper submissions are no longer accepted for most companies. This means you will either need access to commercial filing software or the support of an accountant who files on your behalf.
Who Is Legally Responsible?
This is where many directors are caught off guard. Under UK company law, the directors of a limited company are responsible for ensuring that all statutory filings are made correctly and on time. This includes both the annual accounts filed with Companies House and the Company Tax Return filed with HMRC.
Appointing an accountant does not transfer that legal obligation. If a filing is late or incorrect, HMRC will pursue the company for any resulting penalties or unpaid tax. For sole directors of small limited companies, this responsibility is particularly concentrated, with no compliance department monitoring deadlines for you.
Key Deadlines Every Director Must Know
There are two separate deadlines for Corporation Tax, and conflating them is one of the most common mistakes directors make.
The Payment Deadline
Corporation Tax must be paid within nine months and one day after the end of your company’s accounting period. For a company with a year end of 31 March 2025, the payment deadline would be 1 January 2026. HMRC does not send an invoice.
You are expected to calculate what the company owes and arrange payment before the deadline, even if the CT600 has not yet been finalised.
The Filing Deadline
The CT600 must be filed within 12 months of the end of the accounting period it covers. Using the same example, if your year end is 31 March 2025, the filing deadline is 31 March 2026. The payment deadline falls three months before the filing deadline.
Many directors assume they pay when they file, but this is incorrect, and the gap between the two dates is where late payment interest frequently accumulates.
New companies should also be aware that registration for Corporation Tax must happen within three months of starting to trade. Penalties can begin accumulating even before the first return is due if this registration is missed.
What Happens If You Miss a Deadline
HMRC applies automatic penalties for late CT600 filings with no warnings and no grace period. A return filed even one day late attracts an immediate £100 penalty. If it remains outstanding three months after the deadline, a further £100 is added.
After six months, HMRC may estimate the company’s tax liability and apply an additional penalty of 10% of the unpaid tax. If the return is still not filed after twelve months, another 10% penalty is charged. For companies that are persistently late, the fixed £100 penalties increase to £500 each once a company has been late three times in a row.
These are filing penalties only. Late payment of Corporation Tax incurs daily interest from the date the payment was due, currently set at the Bank of England base rate plus 2.5%. When you combine late filing penalties with late payment interest, the total cost of missing deadlines can become significant for companies that owe a material amount of tax.
Corporation Tax Rates and How They Affect Your Return
Since April 2023, the UK has operated a tiered Corporation Tax system. Companies with taxable profits of £50,000 or less pay the small profits rate of 19%. Those with profits exceeding £250,000 pay the main rate of 25%. Companies with profits between these two thresholds may qualify for Marginal Relief, which effectively creates a sliding scale between the two rates.
These thresholds are divided by the number of associated companies your business has, which means directors with more than one company need to be careful about which rate applies. Getting the rate wrong on your CT600, or failing to claim Marginal Relief when entitled to it, directly affects the amount of tax your company pays.
This is one of the areas where professional input adds clear value, because the calculation is straightforward in principle but easy to get wrong when accounting periods are short or the company structure is complex.
Common Misunderstandings That Cost Directors Money
Even experienced directors can fall into traps that lead to unexpected tax bills or penalties. These are some of the most common mistakes we see, and each one is entirely avoidable with the right guidance.
1. Assuming accounts filed means tax return done
Filing your annual accounts with Companies House and filing your Company Tax Return with HMRC are two entirely separate processes with different deadlines. Companies House acceptance tells you nothing about whether your CT600 has been submitted, and directors who confuse the two frequently end up with penalties they did not see coming.
2. Thinking a loss means no filing obligation
Your company must file a CT600 even if it made a loss. Correctly reporting a loss is important because it can be carried forward to offset against future profits, reducing your tax bill in later years. Failing to file because you assume no profit means no return is a guaranteed route to a penalty.
3. Overlooking the director’s loan account
If you owe money to or from your company through a director’s loan account, these balances can have Corporation Tax implications.
An overdrawn director’s loan account at the end of the accounting period may trigger a Section 455 tax charge, which your company must pay alongside its Corporation Tax. Many directors are unaware of this until it appears on their tax computation.
4. Relying on estimated figures without following up
Some directors submit a CT600 based on estimated figures with the intention of amending it later. Whilst HMRC does allow amendments within 12 months of the filing deadline, relying on estimates can cause problems if the actual figures differ materially.
Underpaid tax attracts interest, and overpaid tax ties up cash unnecessarily. Getting it right first time is always the better approach.
What Your CT600 Filing Actually Includes
A complete Company Tax Return submission involves more than filling in a single form. HMRC expects your company’s statutory accounts for the period, a Corporation Tax computation showing the adjustments from accounting profit to taxable profit, and the CT600 form itself summarising the key figures and any claims for reliefs or allowances.
The statutory accounts and computations must be tagged in iXBRL format for electronic submission. This is a technical requirement that most directors cannot handle without specialist software or professional support. Your limited company accountant will prepare all three elements as a single package, ensuring they are consistent and compliant before submission.
Why Professional Support Is About Risk, Not Convenience
Many directors view accountancy fees as the cost of outsourcing paperwork. In reality, the value of professional support with your Company Tax Return is about reducing the risk of penalties, missed reliefs, and HMRC enquiries.
A qualified accountant will ensure your CT600 is filed correctly and on time, claim all reliefs your company is entitled to, calculate the correct rate including Marginal Relief, and flag potential issues before they become problems.
A single missed deadline can result in penalties that exceed a year’s accountancy fees, and an HMRC enquiry triggered by inconsistencies in your return is far easier to avoid than to resolve. For directors who want to take a proactive approach to year-end tax planning, working with an accountant throughout the year can help reduce your Corporation Tax liability before the year end, not after.
Steps to Stay on Top of Your CT600 Obligations
Staying compliant does not require complex systems, but it does require attention. There are several practical steps you can take to reduce the risk of missed deadlines or errors.
First, confirm your company’s accounting period end date and note both deadlines: the payment deadline at nine months and one day, and the filing deadline at twelve months. Do not assume these are the same as your Companies House deadlines.
Second, keep your bookkeeping up to date throughout the year. Directors who leave everything until year end increase the chance of errors and delays. Monthly bookkeeping gives your accountant clean data and reduces the time needed to prepare your return.
Third, track your director’s loan account, dividends, and salary payments as you go. These are the areas most likely to cause unexpected tax charges, and they are far easier to manage in real time than to untangle retrospectively.
Finally, set aside a Corporation Tax provision during the year so the cash is available when the payment deadline arrives. Treating Corporation Tax as something you pay from whatever is in the account at the time is a recipe for cash flow problems.
Get Expert Support With Your Company Tax Return
Your CT600 is not something to leave to chance. eAccounts provides dedicated company tax return services that take the guesswork out of Corporation Tax filing. Our team of Chartered Accountants handle everything from preparation through to submission, so you can focus on running your business without worrying about penalties or missed reliefs.
Ready to get started? Get an instant quote in under two minutes, call us on 029 2105 5592, or find an accountant near you to speak with our team.
FAQs
What is a CT600?
A CT600 is the official Company Tax Return form that UK limited companies use to report their taxable profits and calculate their Corporation Tax liability to HMRC. It must be submitted electronically along with your statutory accounts and a tax computation.
Do I need to file a CT600 if my company is dormant?
If HMRC has issued a notice requiring a return, yes. Dormant companies that ignore these notices will receive automatic penalties. You can either file a dormant company return or formally notify HMRC that the company is inactive.
When is my Company Tax Return due?
Your CT600 must be filed within 12 months of the end of your accounting period. Your Corporation Tax payment is due earlier, at nine months and one day after the period ends. These are two separate deadlines, and missing either one carries its own penalties or interest charges.
What happens if I file my CT600 late?
HMRC charges an automatic £100 penalty for filing even one day late. A further £100 is added after three months. After six months, HMRC may estimate your tax and charge 10% of the unpaid amount. Companies that are late three times in a row face increased fixed penalties of £500 each.
Can I amend my CT600 after submitting it?
Yes. HMRC allows amendments within 12 months of the original filing deadline. However, if your amended figures show additional tax owed, interest will apply from the date the payment was originally due.
What Corporation Tax rate does my company pay?
Companies with taxable profits of £50,000 or less pay 19%. Those with profits over £250,000 pay 25%. If your profits fall between these thresholds, Marginal Relief may apply, creating an effective rate between the two.
Is filing with Companies House the same as filing with HMRC?
No. Your annual accounts go to Companies House and your Company Tax Return goes to HMRC. They have different deadlines and different requirements. Completing one does not satisfy the other.
