# What is the penalty for a late Company Tax Return? £200 on day one, and how it reaches £4,000 on £18,000 of unpaid tax

> A Company Tax Return is due 12 months after the period ends. The £200, £200, 10% and 10% late penalties on GOV.UK, worked for one client.

- Page to cite: https://aeolookup.com/accountants/company-tax-return-late-filing-penalties-ct600-accountants/
- Publisher: AEO Lookup (https://aeolookup.com/)
- Section: Accountancy practices, Deadlines and penalties
- Published 6 October 2026
- Key fact: GOV.UK, read on 6 October 2026, lists £200 for a return one day late, another £200 at 3 months, 10% of the unpaid tax at 6 months and another 10% at 12 months. For a company with a 31 March 2026 year end and £18,000 of Corporation Tax unpaid, the return is due by 31 March 2027 but the tax by 1 January 2027, and a return filed more than a year late carries £200 + £200 + £1,800 + £1,800 = £4,000 in penalties. If the return has been late three times in a row, each £200 becomes £1,000, and the same case costs £1,000 + £1,000 + £1,800 + £1,800 = £5,600.
- Sources: GOV.UK "Company Tax Returns: deadline and penalties" and "Penalties for late filing of a Company Tax Return" (company-tax-returns pages), read 2026-10-06. The page shows no last updated date in the text read. Exact day counts for the 3, 6 and 12 month steps were not stated on the pages read and are not asserted here.

A Company Tax Return that is one day late costs £200, on GOV.UK's penalty page read on 6 October 2026. A second £200 follows at three months. At six months and at twelve months HMRC adds 10% of the unpaid tax each time. For a company with £18,000 of tax unpaid, a return filed more than a year late carries £4,000 in penalties, before any interest.

## The two dates are not the same date

GOV.UK gives two different deadlines for a company:

- the return is due 12 months after the end of the accounting period it covers;
- the Corporation Tax is usually due 9 months and one day after the end of the accounting period.

For a company with a 31 March 2026 year end, that is 1 January 2027 for the tax and 31 March 2027 for the return. The tax is due three months before the return. A client who waits for the return before paying is already late paying, and the page's late payment consequences are separate from the filing penalties below.

## The penalty table

| How late | Penalty on GOV.UK |
|---|---|
| 1 day | £200 |
| 3 months | Another £200 |
| 6 months | 10% of the unpaid tax |
| 12 months | Another 10% of any unpaid tax |

The page adds one rule for repeat offenders: if the return is late three times in a row, the £200 penalties are increased to £1,000 each. The 10% penalties are not changed.

## One company, worked through

A company has a 31 March 2026 year end and owes £18,000 of Corporation Tax. It files nothing and pays nothing.

| Step | Working | Running total |
|---|---|---|
| Return one day late | £200 | £200 |
| Still not filed at 3 months | £200 | £400 |
| Still not filed at 6 months | 10% of £18,000 = £1,800 | £2,200 |
| Still not filed at 12 months | 10% of £18,000 = £1,800 | £4,000 |

If it was the third late return in a row, the two flat penalties are £1,000 each: £1,000 + £1,000 + £1,800 + £1,800 = £5,600. That is £1,600 more for the same tax.

The 10% penalties depend on the unpaid tax, so paying early lowers them. If the client paid £12,000 by the due date and £6,000 remained unpaid, each 10% penalty would be £600 and the four-step total £1,600. We have worked that from the GOV.UK wording that the penalty is 10% of the unpaid tax. We did not find a page that sets out how HMRC measures "unpaid" at each date, so check it before quoting a figure to a client.

## HMRC's determination at six months

GOV.UK says that when the return is six months overdue, HMRC sends a tax determination letter giving its estimate of the Corporation Tax owed. The page states plainly that the company cannot appeal against it. The company has to settle the tax and file the return, after which HMRC recalculates the interest and penalties.

The practical point is that an estimate can be higher than the real figure, so the quickest way to shrink it is to file.

## Appealing

GOV.UK says a company with a reasonable excuse can appeal online. Three things to know before starting:

1. the company must file the return before appealing the late filing penalty;
2. the online form cannot be saved and returned to, so gather everything first;
3. the form asks for the Unique Taxpayer Reference, the date of the penalty notice, the penalty amount, the accounting period end date and papers explaining the delay.

We did not confirm the appeal time limit for this penalty from the pages read. Our guide on [how long you have to appeal an HMRC penalty](https://aeolookup.com/accountants/how-long-to-appeal-hmrc-penalty/) covers the limits we did read, which come from the Self Assessment rules, so do not assume they transfer.

## Companies House is a separate clock

A late Company Tax Return is an HMRC matter. Late annual accounts at Companies House carry their own penalties, and our [Companies House late filing penalty calculator](https://aeolookup.com/accountants/companies-house-late-filing-penalty-calculator/) works those out. A company can be late with both, and the two fines are not offsets of each other.

## What the practice diarises

1. **Tax due date, per company:** period end plus 9 months and one day.
2. **Return due date:** period end plus 12 months.
3. **A flag for a company that was late last year,** because the £1,000 rule turns on three in a row.
4. **The records chase** that precedes both. Our [records request letter template](https://aeolookup.com/accountants/records-request-letter-template-accountants/) gives each request a date.

Overlyft ![](https://aeolookup.com/img/d/ours-badge.png) can set up an AI assistant that reads the practice's client list, works out the tax and return dates for every company year end and sends the partner a weekly list of returns still waiting for records. The accountant files and signs.

This is general information, not professional advice.

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General information for UK small firms, not legal, tax or financial advice.