Every limited company in the UK must file a corporation tax return after each accounting period, whether it made a profit or not. This guide walks through the deadlines, the CT600 form, what information you need to gather, and how to decide whether to file yourself or use an accountant.

Why your corporation tax return matters more than most directors realise

Every UK limited company must file a corporation tax return with HMRC after each accounting period ends, regardless of whether the company made a profit. The return covers your company’s taxable income, any allowances or reliefs you are claiming, and the amount of Corporation Tax owed. Filing late or filing incorrectly both carry automatic penalties.

HMRC research into SME experiences found that 57% of small businesses rely on a tax agent to a great extent when managing Corporation Tax. A further 19% reported finding the administration genuinely difficult. That is not a surprise given how many moving parts are involved.

Where most limited company directors go wrong

HMRC’s own research confirms that completing and filing the CT600 is consistently ranked among the most difficult corporation tax tasks for SME directors. The problems are rarely dramatic. They are usually quiet errors that compound over time.

Confusing the payment deadline with the filing deadline

These are two different dates and mixing them up is one of the most common causes of penalties. Your Corporation Tax payment is due 9 months and 1 day after your accounting period ends. Your CT600 return is not due until 12 months after your period ends. Paying late costs you interest. Filing late costs you a penalty on top of that.

Claiming reliefs and allowances incorrectly

HMRC notes that Corporation Tax becomes significantly more complex once reliefs or capital allowances are involved. Directors often either miss allowances they are entitled to, or claim them incorrectly, which can attract a compliance check from HMRC. Getting a relief wrong is not necessarily treated as deliberate fraud, but it does create correspondence, delays, and sometimes penalty charges.

How to file a corporation tax return, step by step

The CT600 is filed online using HMRC’s Corporation Tax filing service or compatible accountancy software. You will need to file the CT600 alongside your statutory accounts and a tax computation. Here is the sequence most directors follow.

1. Confirm your accounting period end date. This is the date your company’s financial year closes. It is shown on your confirmation statement from Companies House and on any previous HMRC correspondence. Everything else is calculated from this date, including your payment and filing deadlines.
2. Prepare your statutory accounts and tax computation. Your accounts record income, expenditure, and profit or loss. Your tax computation starts from that profit figure and adjusts it for HMRC purposes, adding back non-deductible costs and applying any allowances or reliefs. For most small companies, this means filing FRS105 micro-entity accounts alongside the CT600, which is the standard for companies with turnover under £632,000.
3. Submit the CT600 online via HMRC’s filing service or your accountant’s software. You will need your company’s Unique Taxpayer Reference (UTR), your Government Gateway login, and the completed accounts and tax computation. If your company has foreign currency transactions, note that HMRC’s free filing service does not support these, and you will need specialist software or professional help.

It is also worth noting that HMRC is actively modernising how company tax returns are filed. A consultation published in March 2026 proposes prescribed content, standardised formats, and XBRL tagging for Corporation Tax computations, with a collaborative development period running from April to September 2026. Changes are unlikely to affect the current filing cycle, but it is something directors and their accountants should be aware of.

DIY or use an accountant: what it actually costs

You can file your own corporation tax return using HMRC’s free online service. Whether that makes sense depends on how straightforward your company’s finances are. A single-director company with one income stream, no employees, no assets, and turnover under the micro-entity threshold is broadly manageable to file yourself, though expect to spend three to five hours the first time. Once your company has employees, multiple revenue streams, capital allowances, or any director loans in play, the risk of error rises sharply and the cost of getting it wrong typically exceeds the cost of an accountant.

How to get moving on your corporation tax return today

The single most useful thing you can do right now is find your accounting period end date and work out how much time you have. Everything else follows from that. If you are within four months of your payment deadline, treat this as urgent. If you have more time, you can move at a steady pace.

• Log in to your Companies House account and check your confirmation statement for your accounting period end date. Calculate your tax payment deadline (9 months and 1 day after period end) and your CT600 filing deadline (12 months after period end).
• Gather your income records, bank statements, and any expenses you plan to claim. If you use bookkeeping software, export your profit and loss report for the period. If your records are incomplete, that is the first thing to fix before attempting to file.