HMRC Corporation Tax determination: what to do and how to replace it

In short: An HMRC Corporation Tax determination is HMRC's own estimate of the tax a company owes, sent when its Company Tax Return (CT600) is about 6 months late. You can't appeal against it; you pay the tax due and file the return, which replaces the estimate if it arrives in time (GOV.UK: penalties for late filing). Taxley prepares and files the overdue CT600 for £49.95 with micro-entity accounts, then files the accounts at Companies House once the return is paid and finished.

Take the 30-second check to see whether Taxley fits your company, or work out your own dates with the Corporation Tax deadline calculator.

Key facts (checked on 4 October 2026)

What is an HMRC Corporation Tax determination?

A Corporation Tax determination is HMRC's own figure for the tax a company owes for a period when the company hasn't delivered a Company Tax Return it was asked for. An HMRC officer sets it "to the best of his information and belief" (FA 1998 Sch 18 para 36), so it is an estimate, not a figure from the accounts.

HMRC's manual says a determination can include estimated profits, relief for losses brought forward, and the rate of tax HMRC thinks is due (HMRC manual CTM95305). The notice must state the date the determination is issued, so keep the letter: that date starts one of the two clocks for replacing HMRC's figure. A determination is only possible after HMRC has served a notice to deliver a Company Tax Return, because the power applies when no return is delivered in response to one; our guide to the CT603 notice to deliver explains that letter.

When does HMRC send a Corporation Tax determination?

Usually about 6 months after the filing deadline, 18 months after the accounting period ends. HMRC can make one at any time after the filing date passes without a return, but its manual says officers normally review a case 18 months after the period ends, unless tax is at risk (HMRC manual CTM95340).

The filing deadline is normally 12 months after the end of the accounting period (GOV.UK: Company Tax Returns), or 3 months after HMRC served its notice if that is later (FA 1998 Sch 18 para 14). HMRC's COTAX manual says officers routinely review cases for a determination at the tax-related penalty stage (HMRC manual COM23170), and GOV.UK's penalty table pairs the two: at 6 months late, HMRC estimates the bill and adds a penalty of 10% of the unpaid tax. There is an outer limit as well: no determination may be made more than 3 years after the day the power becomes exercisable (HMRC manual CTM95390).

Can you appeal against a Corporation Tax determination?

No. GOV.UK says you can't appeal against a tax determination: you must pay the Corporation Tax due and file the return, and HMRC then recalculates the interest and penalties (GOV.UK: penalties for late filing). HMRC's own manual says the same, and adds that the company can replace it by filing its return (HMRC manual COM23170).

HMRC's COTAX manual lists "a revenue determination of tax liability in the absence of a return" among the things a company cannot appeal against (HMRC manual COM10001). Formal postponement of the tax isn't available either, because HMRC's appeals guidance says there must be a valid appeal before tax may be postponed (HMRC manual ARTG2510). The late filing penalties are a different matter. A company with a reasonable excuse can appeal those, but GOV.UK says the Company Tax Return must be filed first (GOV.UK: penalties for late filing), and there are usually 30 days from the date the penalty was issued (GOV.UK: disagree with a penalty). Either way, the return comes first.

How do you replace a Corporation Tax determination?

File the company's Company Tax Return for the period. When the company delivers a return for an accounting period ending in the period the notice covered, the self-assessment in that return supersedes HMRC's determination (FA 1998 Sch 18 para 40). The return's figures then replace HMRC's estimate, provided it arrives within the time limit.

The return has to be a real one: every Company Tax Return includes a declaration that it is correct and complete to the best of the signer's knowledge (FA 1998 Sch 18 para 3), so the figures come from the company's accounts, not a guess made to beat HMRC's estimate. The periods don't have to match HMRC's exactly: where HMRC made determinations for more than one period, the return replaces the one it overlaps most (HMRC manual CTM95400). If the company shows it had no accounting period ending in the notice period, had already delivered the return, or had no return yet due, the determination has no effect and HMRC should discharge it (HMRC manual CTM95330). Recovery proceedings already started can continue only for the tax due under the company's own return that is still unpaid (FA 1998 Sch 18 para 40).

How long do you have to replace a determination?

Until the later of two dates: 3 years after the day HMRC's power to make the determination first became exercisable, or 12 months after the date of the determination. A return made after both dates doesn't replace it (FA 1998 Sch 18 para 40); HMRC's manual sets out the same rule (HMRC manual CTM95400).

In practice the 3-year clock starts the day after the filing date. The power becomes exercisable if no return is delivered on or before the filing date, and HMRC's manual says a determination can be made at any time after that date (HMRC manual CTM95310). The 12-month clock runs from the date of the determination, which the notice must state (FA 1998 Sch 18 para 36). The second clock matters when HMRC makes its determination late, close to its own 3-year limit: a determination issued 2 months before that limit still leaves the company 12 months from its date.

If both dates have passed, the determination stands. The law keeps one narrow route, "special relief", a claim that the determined tax isn't due (FA 1998 Sch 18 para 51BA). HMRC doesn't have to give effect to it unless it would be unconscionable for HMRC to collect the amount, the company's tax affairs are otherwise up to date or being brought up to date, and the company hasn't relied on special relief before, unless its circumstances are exceptional. Those conditions are much narrower than filing in time, and a claim is one to take professional advice on.

What does a determination timeline look like for a 31 March year end?

For a 12-month accounting period ending on 31 March 2025, the filing date was 31 March 2026, HMRC's power to make a determination began on 1 April 2026, and a determination dated 15 October 2026 can be replaced by a return made up to 1 April 2029. A later determination can push that last date further out.

Taxley's calculated timeline: accounting period 1 April 2024 to 31 March 2025 (assumes the notice to deliver was served in good time and no return filed)

The arithmetic: tax is due 9 months and 1 day after the period ends, and 31 March 2025 plus 9 months is 31 December 2025, so 1 January 2026 (GOV.UK: Pay your Corporation Tax bill). The filing date is 12 months after the period ends, 31 March 2026. With no return by then, the power to determine began the next day, 1 April 2026, and 3 years later is 1 April 2029. For a determination dated 15 October 2026, 12 months later is 15 October 2027, earlier than 1 April 2029, so 1 April 2029 is the last day. For a late determination dated 1 February 2029, 12 months later is 1 February 2030, after 1 April 2029, so the company has until 1 February 2030 (FA 1998 Sch 18 para 40).

Don't plan around the last day, because the penalty dates come much sooner. A return delivered on or before 31 March 2027, 2 years after the period ended, keeps the tax-related penalty at 10% of the unpaid tax; after that it is 20% (FA 1998 Sch 18 para 18). The filing date fell just before 1 April 2026, so the flat penalty uses the old amounts, £100, or £200 if the return is more than 3 months late (£500 and £1,000 for a third late return in a row); for filing dates from 1 April 2026 they are £200 and £400 (£1,000 and £2,000 for a third in a row) (GOV.UK: increases to Corporation Tax late filing penalties). Taxley's free late filing penalty calculator works out both penalties and the interest for a single period.

Do you have to pay the tax in a determination?

Yes, unless and until the company's return replaces it. A determination has effect for payment, collection, recovery, interest and tax-related penalties as if it were the company's own self-assessment (FA 1998 Sch 18 para 39), and GOV.UK says you must pay the Corporation Tax due and file the return (GOV.UK: penalties for late filing).

Paying stops late payment interest on whatever is paid, because interest runs from the day after the tax was due until the date it is paid (GOV.UK: Corporation Tax interest charges). If HMRC's estimate turns out higher than the real tax, the return claims the difference back: the CT600 has a box for tax already paid and not repaid, box 595 (HMRC: The Company Tax Return guide). GOV.UK says HMRC may first use a repayment to pay other tax the company owes or a late filing penalty (GOV.UK: Get a refund or interest on your Corporation Tax). If HMRC's estimate is lower than the real tax, interest runs on the full amount from the original due date.

If the company can't pay, HMRC may agree a monthly payment plan when it thinks the plan is affordable; otherwise it asks for the full amount (GOV.UK: If you cannot pay your tax bill on time). Pay with the 17-character Corporation Tax payment reference for that accounting period, which is on the notice to deliver and in the company's HMRC online account and changes with each period (GOV.UK: Pay your Corporation Tax bill). Our guide on how to pay Corporation Tax covers the payment methods. A payment plan doesn't replace the return: the determination stays in place until the company files.

What happens if you ignore an HMRC determination?

HMRC can collect the determined tax as if the company had self-assessed it. GOV.UK says that if you don't contact HMRC or can't agree a payment plan, HMRC may use a debt collection agency, take you to court or close down the company, among other steps (GOV.UK: If you do not pay).

The same page lists taking money directly from bank accounts in England, Wales and Northern Ireland, and says HMRC will tell you before acting. Meanwhile interest on the unpaid tax runs every day, at 7.75% a year from 9 January 2026 (HMRC interest rates). The tax-related penalty rises from 10% to 20% of the unpaid tax once the return arrives more than 2 years after the period ends (FA 1998 Sch 18 para 18). Once both time limits have passed, the company can no longer replace HMRC's figure with its own, however far the estimate is from the real tax. Several years with no returns can each carry a determination; which overdue CT600 to file first sets out the order.

Can Taxley file the return that replaces an HMRC determination?

Yes, for older years back to accounting periods that started on 1 April 2020. Taxley (taxley.co.uk), UK online software that prepares and files the Company Tax Return (CT600) with HMRC and the annual accounts with Companies House, builds the overdue return from the company's own figures and files it with HMRC using the company's own Government Gateway user ID.

Taxley works the return out from the company's accounts, not from HMRC's estimate. If the company has already paid HMRC's determined amount, enter it under "Corporation Tax already paid" (box 595). When that payment is more than the tax on the return, Taxley claims the overpayment back (box 865) and ticks "a repayment is due for this period" (box 40) for you. The fee is £49.95 with micro-entity accounts, £139 with small-company accounts, or £14.95 for a dormant company's nil return. It includes filing that year's accounts at Companies House once the return is paid and the accounts are finished, using the company's 6-character authentication code; if Companies House already has accounts for that period, Taxley files the CT600 only.

Taxley is software, not an accountant or tax agent, and it doesn't give tax advice. It doesn't read HMRC's determination, correspond with HMRC for you or pay the tax, and it doesn't check whether your return is still within the time limit to replace the determination, so work out that date from the notice first. Periods that started before 1 April 2020 have to be filed another way. Take the 30-second check to see whether Taxley fits your company, or see the prices.

Frequently asked questions

Does paying the determined amount mean the company doesn't have to file?

No. GOV.UK says you must pay the Corporation Tax due and file the return (GOV.UK: penalties for late filing). Paying stops interest on the amount paid, but only the company's own return replaces HMRC's estimate with the real figure.

What if HMRC's determination is lower than the tax the company really owes?

Filed within the time limit, the company's return replaces it with the higher figure. Interest is charged on Corporation Tax that is paid late or underpaid from the day after it was due (GOV.UK: Corporation Tax interest charges), so paying only the determined amount leaves interest building on the rest.

What if the company was dormant for the period?

A company that has received a notice to deliver a Company Tax Return still has to file one, which shows HMRC it was dormant for that period (GOV.UK: Dormant for Corporation Tax). If the return shows no accounting period ended in the notice period, HMRC's manual says the determination is of no effect (HMRC manual CTM95400).

Where do I find the date of the determination?

On the notice itself. HMRC must serve notice of a determination on the company, stating the date on which it is issued (FA 1998 Sch 18 para 36). That date starts the 12-month period for replacing the determination with a return.

General information, not personalised tax or accounting advice.