How to File a CT600 Without an Accountant in 2026

HMRC's free filing service has closed. But that does not mean you need to hire an accountant. Many directors file their own Corporation Tax return every year. Here is exactly how to do it.

Can you still file your own Corporation Tax return?

Yes. Many directors assume that filing a CT600 now requires an accountant because HMRC's free filing service has closed. That is not the case.

You can still file your own Corporation Tax return provided you use compatible software and your company's affairs are relatively straightforward. The obligation has not changed. Only the tool you use to meet it has.

What is a CT600?

A CT600 is the Corporation Tax return submitted to HMRC by UK limited companies. It tells HMRC your income, expenses, profits or losses, and the amount of Corporation Tax due. Most limited companies must submit one, even if they made little profit or no profit at all.

The CT600 must be accompanied by your statutory accounts and a tax computation, both formatted in iXBRL. Most filing software handles this automatically.

Corporation Tax rates in 2026

The rate your company pays depends on its taxable profits for the accounting period:

Most micro-entities and small owner-managed companies fall into the small profits rate band, making the calculation straightforward.

What information do you need?

Before starting, gather:

• Company details and Unique Taxpayer Reference (UTR)
• Accounting period start and end dates
• Profit and loss information
• Balance sheet figures
• Corporation Tax calculations
• Previous year figures where relevant

If you maintain bookkeeping records throughout the year, most of this information should already be available.

Filing your CT600: step by step

Step 1: Prepare your accounts

Your annual accounts form the basis of your tax return. For a typical micro-entity, these usually include a profit and loss account, a balance sheet, and notes to the accounts.

Step 2: Calculate Corporation Tax

Once the accounts are complete, calculate your taxable profit, any adjustments required by HMRC, and the Corporation Tax due. Filing software generally performs much of this calculation automatically, including adding back disallowable expenses and applying capital allowances.

Step 3: Generate iXBRL accounts

HMRC requires accounts to be submitted in iXBRL format. This is a structured digital format that allows HMRC systems to read the information automatically. PDFs are not accepted. Most filing software creates this format for you without any technical knowledge required.

Step 4: Submit your CT600

The final submission normally includes the CT600 return, your iXBRL accounts, and supporting computations. You will need your Government Gateway account linked to your company's UTR to submit. Once submitted, HMRC provides a confirmation receipt. Keep a copy.

Filing and payment deadlines

These are two separate deadlines that are easy to confuse:

• Payment deadline: 9 months and 1 day after the end of your accounting period
• Filing deadline: 12 months after the end of your accounting period

Note that Corporation Tax must be paid before the return is due. Do not wait until you have filed to arrange payment.

Common mistakes to avoid

Using incorrect accounting period dates

Many filing rejections happen because the CT600 dates do not match the company's actual accounting period. Your first accounting period may differ from subsequent years, particularly if your company was incorporated mid-year.

Missing iXBRL tagging requirements

Mis-tagged accounts are the most common cause of immediate rejection by HMRC. Line items must align exactly and the taxonomy must be current. Always use up-to-date software rather than trying to create iXBRL files manually.

Not retaining records

Keep copies of submitted returns, accounts, tax calculations, and submission confirmations. HMRC can open an enquiry into a return up to 12 months after the filing date, and further back in cases of suspected fraud or serious errors.

Assuming a loss means you do not need to file

A CT600 must be filed even if your company made a loss or has no Corporation Tax to pay. The obligation to file and the obligation to pay are entirely separate.

When should you use an accountant?

For a straightforward owner-managed business, software is often sufficient. Consider professional help if your situation involves:

• Research and development tax credit claims
• Capital allowances on significant assets
• Investment income
• Group company structures
• Overseas activities or a UK branch of a foreign company
• An accounting period longer than 12 months

If none of those apply, you are likely in the territory where self-filing with the right software is practical, affordable, and well within reach.