How to File a CT600 Yourself, Step by Step

Last updated: 15 October 2026 · Written by Axel

Here is how to file a CT600 yourself in the right order: prepare your micro-entity accounts, file them with Companies House, then send your CT600 to HMRC with the accounts and a tax computation attached, and pay any Corporation Tax by its own earlier deadline. For a simple trading company or a dormant one, that is a job most directors can do in an afternoon once the figures are in front of them. The hard part is not the form. It is knowing what goes where, and when.

What a CT600 actually is

The CT600 is HMRC's Company Tax Return. You have to file one if HMRC sends your company a "notice to deliver a Company Tax Return", and per GOV.UK's Company Tax Returns guide that still applies if you made a loss or have no Corporation Tax to pay.

It is not the same as your annual accounts. The accounts show what the company did. The CT600 works out the profit or loss for Corporation Tax, which is usually different from the profit in the accounts (things like depreciation and some expenses get treated differently), and from that, the tax bill.

So one year end produces three things, going to two bodies:

• Annual accounts to Companies House (public)
• The CT600, plus your full accounts and a tax computation, to HMRC (private)
• The Corporation Tax payment to HMRC

The deadlines, from one date

Everything runs off your company's year end. According to GOV.UK's guide to preparing and filing annual accounts:

Take a year end of 31 March 2026. Accounts are due by 31 December 2026, the tax by 1 January 2027, and the CT600 by 31 March 2027.

Notice the trap. The corporation tax return deadline is the latest of the three, so people treat 12 months as "the" deadline. It isn't. You cannot know what tax to pay at 9 months without doing the CT600 sums, so in practice treat 9 months as the deadline for everything.

What to gather before you start

Get these together first. Hunting for a code in deadline week is how returns end up late.

• Your company's Corporation Tax UTR. A 10-digit number on HMRC letters such as your notice to file or payment reminders. Lost it? HMRC will post a copy to the registered office, as explained on the find your UTR page.
• Your Companies House authentication code. A 6-character code that acts like a director's signature for online filings. If you need a new one, it comes by post to the registered office and can take up to 10 working days. Request it early.
• Your Government Gateway login for the company's HMRC business tax account, with Corporation Tax added.
• Bank statements for the whole year, every business account.
• Year-end figures: sales, expenses, money owed to and by the company at the year end, cash at bank, any equipment or vans bought, any director's loan balance, and dividends paid.
• Last year's accounts and CT600, if this is not your first year. The opening figures have to match.
• The 17-character payment reference for the period, from your notice to deliver or HMRC reminders, for when you pay.

How to file a CT600 yourself, in order

1. Check you qualify as a micro-entity

Micro-entity accounts are the simplest kind, and they are what makes doing this yourself realistic. For accounting periods starting on or after 6 April 2025, your company is a micro-entity if it meets at least 2 of these, per GOV.UK's micro-entity rules:

• turnover of £1 million or less
• £500,000 or less on the balance sheet
• 10 employees or less

Most one or two person trading companies are comfortably inside that.

2. Prepare the accounts

Work out the profit and loss for the year and the balance sheet at the year end from your bank statements and records. Micro-entities can send Companies House a short balance sheet only, but HMRC still gets full statutory accounts with the return. If you want to understand what does and does not go on the public register, read our guide to filleted accounts.

3. File the accounts with Companies House

Submit them with your authentication code. Do it as soon as the figures are settled, well inside the 9 months.

4. Prepare the computation and CT600

Start from the accounting profit, add back things that are not allowed for tax (depreciation, client entertaining, fines), claim capital allowances on equipment instead, and arrive at the taxable profit. The CT600 takes that figure and calculates the tax.

5. Send the CT600 to HMRC

The CT600, the computation and the accounts go together, in HMRC's iXBRL format, through software logged in with your Government Gateway. Keep the submission receipt.

6. Pay the Corporation Tax

Pay by 9 months and 1 day after the period end using the 17-character reference. HMRC's pay Corporation Tax page says Faster Payments usually arrive the same or next day, Bacs takes 3 working days, and you cannot pay by post. If there is nothing to pay, the CT600 is how you tell HMRC that.

Ways to file: what is actually open to you

HMRC's free service for filing company accounts and tax returns closed on 31 March 2026. From 1 April 2026 you need commercial software to file a Company Tax Return online, and paper CT600s are only accepted with a reasonable excuse or if you are filing in Welsh. Companies House WebFiling still takes accounts for now, but from 1 April 2028 accounts can only be filed through software.

Our honest view: if your only year-end job is accounts plus CT600, paying a monthly subscription for it is overkill. Pick something that files both from the same figures, because mismatched accounts between Companies House and HMRC are a common, avoidable mess.

Who should do this themselves, and who shouldn't

Doing it yourself suits:

• a one or two director company with straightforward trading income and costs
• a company with no employees beyond the directors, or a simple payroll
• a dormant company, where the accounts are short and, once HMRC has been told the company is dormant, a CT600 is only needed if HMRC sends a further notice

Get your own accountant if any of these apply:

• you are part of a group or have associated companies
• you want to claim R&D tax relief
• you have losses to carry back or across, property income, or overseas income
• several years are unfiled, or HMRC has issued an estimated tax determination
• the business is past the micro-entity limits

That is not modesty. Those are the situations where a wrong figure costs far more than a professional's fee.

Mistakes that trigger penalties

• Working to the 12-month date. The Companies House deadline at 9 months comes first, and that penalty starts at £150.
• Requesting the authentication code too late. Up to 10 working days by post can swallow your margin.
• Thinking "no tax due" means "no return". If HMRC sent a notice, the CT600 is due anyway. A late return costs £200 at one day late, with more at 3 months.
• Paying with the wrong reference, which can leave the money sitting against the wrong bill.
• Accounts and CT600 that disagree, usually from filing them in two places at different times.

The full list of fines, and how appeals work, is in our guide to late filing penalties from Companies House and HMRC. Our Corporation Tax filing deadlines page works out your own dates.

Axel

Full-stack developer and founder of AXEL.TRADE, with 10+ years running businesses. Based in Manchester, building websites and the A.X.E.L platform for tradesmen and small businesses.