What is a corporation tax return? It sounds like it should be obvious, but most directors I speak to are genuinely hazy on what it involves, when it’s due, and whether getting it wrong will land them in trouble.

What a Corporation Tax Return Actually Is

A corporation tax return is the document your limited company sends to HMRC to report its profits and calculate how much Corporation Tax it owes. The form itself is called a CT600. You file it online through HMRC’s systems, and it has to be accompanied by your company accounts and a tax computation showing how you arrived at the figure.

Every limited company registered in the UK has to file one, even if it made no profit that year. If your company made a loss, you still need to file a return and tell HMRC about it. There are no exceptions based on size or turnover.

The Deadlines You Need to Get Clear on Now

Your accounting period is usually 12 months long, and it typically matches your company’s financial year. If your accounts run from 1 April 2025 to 31 March 2026, your Corporation Tax payment is due by 1 January 2027 and your return must be filed by 31 March 2027. The payment deadline comes first, which catches a lot of directors off guard.

Missing the filing deadline brings an automatic £100 penalty, even if you owe no tax at all. If the return is still outstanding three months later, HMRC adds another £100. After six months, HMRC will estimate your tax liability themselves and charge 10% of whatever unpaid amount they calculate, which is usually set high. HMRC outlines exactly how these compliance checks and penalties work, and the numbers are specific enough that it is worth understanding before a deadline passes.

What Actually Goes Into a Corporation Tax Return

The CT600 form itself asks for your company’s taxable profits, any reliefs or allowances you’re claiming, and the tax calculation. But the form doesn’t stand alone. You also need to attach your statutory accounts (the profit and loss account and balance sheet) and a tax computation that shows how you got from your accounting profit to your taxable profit.

For most small limited companies, the accounts are prepared under FRS 105, the micro-entity reporting standard, which keeps things relatively simple. The tricky part is the tax computation. This is where you add back disallowable expenses, apply capital allowances on equipment, and account for any reliefs you’re entitled to. HMRC is currently consulting on modernising how companies file CT returns, with a proposed shift to standardised XBRL-tagged formats, so the process may look different in the years ahead.

Can You File a Corporation Tax Return Yourself?

Technically, yes. HMRC has a free filing service, and if your company is very straightforward, one director, simple income, no assets being depreciated, no foreign transactions, and a clean set of records, it’s possible to do it yourself. Expect it to take three to five hours the first time, and be aware that HMRC’s free tool has real limitations. Business owners have reported that it does not support foreign currency transactions, for example, which rules it out for a lot of companies right away.

HMRC’s own research found that 19% of small businesses find Corporation Tax administration difficult, and 57% rely on a tax agent to manage it for them. That is not because the other 43% are more capable. It is because the cost-benefit calculation usually favours getting it done properly over spending several stressful evenings on something a professional handles routinely. If your company has more than one income stream, any employees, assets that depreciate, or if you want to claim R&D relief or any other allowance, I’d say get an accountant involved.