How to File a CT600 Yourself: The Complete UK Guide

You can file a CT600 yourself by preparing your accounts, calculating the Corporation Tax due, and submitting your CT600 form and supporting accounts to HMRC. 

The process is more straightforward than you’d think, so you may be able to skip the accountant and keep that cost in your business.

This guide covers registration, taxable profit calculation, the two deadlines, and the 2026 switch to commercial software.

Key points

• Your Corporation Tax payment and filing deadlines are different 📅
  Your tax bill is due nine months and one day after your accounting period ends, but you have 12 months to file your CT600. 
• Your accounting profit isn’t necessarily your taxable profit 🧮
  Before calculating Corporation Tax, you may need to add back disallowable expenses, deduct capital allowances, and apply available reliefs or losses. This is one of the more complicated parts of filing a CT600 yourself.
• Filing yourself works best for straightforward companies ✅
  If your accounts are simple and you have no complex reliefs or tax adjustments, software is likely enough. If you're claiming Research & Development relief, running group companies, or carrying losses, an accountant may be worth the cost.
• ANNA centralises your bookkeeping, tax, and filing 🚀
  ANNA brings your bookkeeping, tax calculations, and filing together in one place, so you can stay organised and take the hassle out of managing your business taxes. 

The 2026 change: You can't file through HMRC's website any more

Until 31 March 2026, HMRC ran a free online service called Company Accounts and Tax Online (CATO) that let small companies enter their figures directly into a government portal. 

That service has closed permanently. From 1 April 2026, every company has to use commercial software to file a CT600, regardless of its size or trading status. 

There's no extension and no free government alternative, though a small number of companies can still file on paper, for example if they’re filing in Welsh or have a reasonable excuse.

This change doesn’t mean you have to hire an accountant. You can still prepare and file your own CT600, but you now need compatible commercial software to do it. 

HMRC publishes a list of recognised providers, ranging from basic tools built specifically for CT600 filing to full accounting packages that include it as a feature. Pricing varies widely, so compare a couple of options against your company's needs before you commit to one.

🧠 Good to know

You don’t have to manage your company’s tax admin alone. ANNA can calculate your taxes, keep your records organised, and help you stay on top of deadlines, so you have less to worry about when it’s time to file. 

Registering for Corporation Tax

Before you can file a CT600, your company needs to be registered for Corporation Tax with HMRC. You need to do this within three months of starting to trade, taking payments, or advertising.

Registering with HMRC is separate from incorporating your company at Companies House, so don't assume one automatically completes the other. If you've recently set up a company, make sure that you've completed both steps.

If your company is dormant, meaning it isn't trading and has no significant transactions, you don't need to file a CT600 once you've notified HMRC. 

However, HMRC may still send you a formal notice to file a return. If that happens, you have to file it even if the company was dormant for the period.

What you need before filing a CT600

Once your company is registered and you're ready to file, gather your documents and figures. You'll typically need:

• Your company's Unique Taxpayer Reference (UTR), the 10-digit number HMRC gives you when you register for Corporation Tax
• The Government Gateway user ID and password linked to the company
• Your finalised annual accounts for the accounting period, including the profit and loss account and balance sheet
• A tax computation showing how you’ve worked out your taxable profit from your accounting profit
• Supporting records such as invoices, bank statements, and details of assets bought during the period

You need to submit your accounts and tax computation in iXBRL format. This is a tagged version of your financial information that HMRC's systems can read. Suitable CT600 software should generate it from your accounts and figures.

Working out your Corporation Tax

Profit shown in your accounts isn't necessarily the same as the profit HMRC will tax you on.

You'll need to adjust your accounting profit to arrive at your taxable profit. Follow these steps:

• Start with your accounting profit: Use your net profit before tax from your annual accounts
• Add back disallowable expenses: Add back costs that aren't deductible for Corporation Tax, such as client entertainment, most fines and penalties, and depreciation
• Deduct capital allowances: Claim tax relief on qualifying assets, such as equipment and vehicles, instead of deducting depreciation
• Apply available reliefs: Include any other reliefs or allowances your company qualifies for, such as Research & Development (R&D) tax relief or losses carried forward
• Calculate Corporation Tax: Apply the relevant rate to your taxable profit: 19% on profits below £50,000 and 25% above £250,000, with Marginal Relief tapering between the two

This is often the trickiest part of filing a CT600 yourself because not every business expense is tax-deductible. Make sure you understand which costs need to be adjusted before submitting your return.

Filing and payment deadlines

There are two different Corporation Tax deadlines:

• Payment deadline: You have to pay your Corporation Tax bill nine months and one day after the end of your accounting period
• Filing deadline: You have 12 months from the end of your accounting period to file your Company Tax Return

This means you’ll usually need to pay your Corporation Tax before you file your return. If you miss the payment deadline, HMRC charges interest on the unpaid tax from the day after the deadline, even if you haven’t yet filed your CT600.

Penalties for late or incorrect returns

For Company Tax Returns with a filing deadline on or after 1 April 2026, HMRC's late filing penalties increase the longer your return remains outstanding:

• One day late: £200 fixed penalty
• Three months late: An additional £200 penalty
• Six months late: An additional penalty of 10% of the unpaid tax
• Twelve months late: Another penalty of 10% of the unpaid tax

Even if your company made a loss and owes no Corporation Tax, filing late still triggers these penalties.

HMRC can also charge separate penalties if your Company Tax Return contains an error. 

The amount depends on factors such as whether the mistake was careless or deliberate and whether it was actively concealed. A genuine error that you take reasonable care to correct may result in little or no penalty, while deliberately understating your profits can lead to a significant charge.

If you discover a mistake after filing your CT600, you can amend the return through your tax software within 12 months of the original filing deadline. After this period, you need to contact HMRC in writing to request a correction.

When filing CT600 yourself makes sense

Filing your own CT600 is a suitable option if your company has a simple setup and straightforward finances, such as one director, basic income and expenses, no complicated tax reliefs, and complete and accurate accounts.

Corporation Tax software can guide you through the return and calculate the tax due, usually for an affordable price. However, you remain responsible for ensuring the figures are correct.

Working with an accountant may be the better option if your company has multiple income streams, R&D claims, losses to carry forward or back, group companies, or complicated tax adjustments. The same applies if you are unsure whether your accounts correctly identify which expenses and deductions HMRC allows.

Picking the cheapest option won’t necessarily save you money.

An accountant may cost more than basic software, but their advice could help you identify capital allowances, tax reliefs or allowable deductions that reduce your overall tax bill. They can also prevent mistakes that can trigger a penalty, such as misclassified expenses.

What ANNA can do for your Company Tax Return

Filing your own CT600 means calculating your taxable profit, keeping the right records, and tracking two deadlines, all on top of running your business.

ANNA brings the whole process into one place, so each step flows into the next instead of living in a separate tool.

Here's how ANNA can help:

• HMRC-recognised CT600 filing: File your Company Tax Return through software that's already on HMRC's approved list, so you don't have to hunt through the providers yourself.
• Automatic iXBRL tagging: You don’t need to worry about mistagged submissions getting rejected – ANNA automatically converts your accounts and computations into the format HMRC's systems require
• Built-in tax calculations: ANNA automatically works out your Corporation Tax bill, including Marginal Relief where it applies
• Deadline tracking: ANNA flags your payment and filing deadlines separately, so you don't accidentally file before you've paid, or miss the payment window
• Real-time bookkeeping: Have your income, expenses, and bank transactions organised throughout the year and ensure your accounts are ready when your accounting period ends
• Free Self Assessment filing: File your Self Assessment for free with ANNA, with built-in support for dividends, self-employed profits, and rental income
• VAT return filing: Keep your VAT records organised and submit VAT returns through HMRC's Making Tax Digital system
• Payroll and PAYE support: Manage payroll and stay on top of your PAYE obligations alongside the rest of your business admin
• 24/7 support: Get help from real professionals whenever a question comes up, day or night

Ready to take the stress out of Corporation Tax? Get started with ANNA today.