If you run a limited company in the UK, Corporation Tax is something you cannot really avoid. At the end of the accounting year, you need to work out how much profit the company has made for tax purposes and report it to HMRC.
For someone who is new to running a company, the process can look more complicated than it actually is. You have your accounts to prepare, tax adjustments to make, a CT600 to complete and, of course, deadlines to remember.
So, how does it all fit together?
Let’s take a look.
What is a Corporation Tax Return?
A Corporation Tax Return is how a company tells HMRC about its Corporation Tax position.
The main form is called a CT600. But the return is not just the CT600 form. It normally includes the company’s accounts, tax computations and any supplementary pages that apply to the business.
The important thing to remember is that the profit in your accounts is not necessarily the same as the profit HMRC will tax.
That’s where the tax calculation comes in.
When does a company need to file one?
If HMRC has asked your company to file a Company Tax Return, you need to submit it for the relevant accounting period.
This still applies if your company has made a loss or doesn’t have any Corporation Tax to pay.
If you’ve recently started trading through a limited company, it’s also important to make sure HMRC knows that the company is active and liable for Corporation Tax.
When is the Corporation Tax Return due?
There are two dates you need to keep separate.
For most companies, Corporation Tax needs to be paid 9 months and 1 day after the end of the accounting period.
The Company Tax Return is normally due 12 months after the end of the accounting period.
For example, if your company’s accounting period ends on 31 December 2026, the Corporation Tax will normally be due on 1 October 2027. The Company Tax Return will then be due by 31 December 2027.
So, you don’t have to wait until you file the return to pay the tax.
How do you actually file a Corporation Tax Return?
The first step is getting your accounts in order.
Start with your accounts
Before looking at the tax return, you need to know what happened financially during the year.
You’ll usually need information such as:
- Sales and other income
- Business expenses
- Bank transactions
- Payroll
- Equipment and other assets
- Loans and interest
- Money owed by customers
- Bills that haven’t been paid yet
If the bookkeeping has been kept up to date during the year, this part is much easier.
If the records are incomplete, it’s worth sorting them out before starting the Corporation Tax calculation. Otherwise, you could end up working with figures that aren’t accurate.
Then work out the taxable profit
This is where Corporation Tax can get a little confusing.
The profit shown in your company’s accounts is an accounting figure. HMRC uses a taxable profit figure, which may be different.
Some expenses that appear in the accounts may not be allowable for tax purposes. There may also be capital allowances, losses or other reliefs that need to be taken into account.
So, you normally start with the accounting profit and make the necessary tax adjustments to arrive at the taxable profit.
That figure is then used to work out how much Corporation Tax the company owes.
Which Corporation Tax rate applies?
For the 2026 financial year, the main Corporation Tax rate is 25%.
Companies with profits of £50,000 or less may generally qualify for the 19% small profits rate. If profits are between £50,000 and £250,000, Marginal Relief may apply.
It’s not always as simple as looking at the company’s profit and choosing a rate, though. Associated companies and the length of the accounting period can affect the thresholds.
This is one of the areas where it’s worth checking the figures carefully rather than making assumptions.
Completing the CT600
Once the accounts and tax calculation are ready, you can prepare the Company Tax Return.
The CT600 asks for information about the company and its tax position. This can include the accounting period, turnover, taxable profits, losses, tax already paid and the Corporation Tax due.
Depending on what the company does, additional information or supplementary pages may also be required.
The figures should match the supporting accounts and tax computation.
How do you submit it?
There has been a change here that companies filing in 2026 need to know about.
HMRC’s online service for filing Company Tax Returns closed on 31 March 2026.
From 1 April 2026, companies generally need to use commercial software to file their Company Tax Returns with HMRC.
If you use an accountant, they will normally have suitable software and can submit the return for you.
For companies handling their own accounts, it’s important to check that the software they use supports Corporation Tax filing.
Don’t forget to pay the tax
Preparing the return is only one part of the job.
You also need to make sure the Corporation Tax is paid on time.
For most companies, this is 9 months and 1 day after the end of the accounting period.
For example, a company with a 31 December year end would normally need to pay its Corporation Tax by 1 October of the following year.
Larger companies can have different payment arrangements, so the standard deadline doesn’t apply to everyone.
What if you make a mistake?
Mistakes can happen, particularly when a company is preparing its first Corporation Tax Return.
Maybe an expense has been treated incorrectly, a tax adjustment has been missed or some information was entered incorrectly.
Don’t simply ignore it.
If you realise that something is wrong after filing, check the appropriate HMRC process for correcting the return. The sooner an error is dealt with, the better.
Late filing can also result in penalties, even where there is little or no Corporation Tax to pay.
A few mistakes businesses often make
There are a few things worth watching out for when preparing a Corporation Tax Return.
Leaving the bookkeeping until the last minute
If your records aren’t up to date, preparing the tax return becomes much harder.
Assuming accounts profit is taxable profit
There can be several tax adjustments between the two figures.
Forgetting about capital allowances or losses
These can affect the amount of taxable profit, depending on the company’s circumstances.
Missing the payment date
The Corporation Tax payment deadline usually comes before the tax return deadline.
Not considering associated companies
Associated companies can affect the Corporation Tax thresholds and should be checked before applying the rates.
Can you file it yourself?
Yes, you can.
If your company is straightforward and you understand the accounts and Corporation Tax requirements, you can prepare and submit the return using appropriate commercial software.
But things can become less straightforward when the company has losses, significant assets, associated companies, overseas transactions or other tax issues.
In those situations, getting an accountant to review the figures can save a lot of time and help avoid mistakes.
Final thoughts
Corporation Tax doesn’t have to be difficult, but it does need to be handled properly.
Keep your bookkeeping up to date, prepare your accounts, check the tax adjustments, work out the correct Corporation Tax and make sure you know both deadlines.
And remember, the date for paying Corporation Tax is usually earlier than the date for filing the return.
If you’re not sure whether your company has calculated its Corporation Tax correctly, it’s better to get it checked before submitting the return.
Need help with UK Corporation Tax?
SAS KPO Services UK supports accounting firms and businesses with Corporation Tax, bookkeeping, VAT returns, payroll, personal tax returns, iXBRL tagging and other UK accounting work. If you need extra support with your UK accounting workload, our team can work alongside you and help with the day-to-day accounting work as well as year-end requirements.