R&D tax relief deadlines: every date, from the legislation

Every deadline in an R&D claim is set by statute, and almost none of them run from the same date. The notification runs from the end of your period of account. The tax return runs from the end of the accounting period. The claim itself runs from the end of the period of account again, but over two years rather than six months. HMRC’s enquiry window runs from the day the return arrives, and discovery from the end of the accounting period. Three different anchors, which is why these dates get rebuilt wrongly from memory.

The short version, for a company with an ordinary twelve-month year: notify within six months of the year end; file the Additional Information Form before or with the claim; deliver the return twelve months after the year end; make or amend the claim within two years of the year end. HMRC’s enquiry window then closes twelve months after the return arrives, and the period can still be reopened by discovery for four years after it ends, on limited grounds — longer if anything went wrong carelessly or deliberately. The rest of this page takes each date from the legislation and works three sets of accounts through it.

Which date does each deadline run from?

Two dates do the work. The period of account is the period your accounts cover. The accounting period is the period your company tax return covers, and it can never run longer than twelve months. For most companies the two are identical. Draw up accounts for longer than twelve months and they part company at once: one set of accounts, two accounting periods, two returns, two claims.

Notification and the claim window are fixed by the period of account. The tax payment date and discovery run from the accounting period. The filing date runs from the accounting period but is pushed out where the accounts end later. So a change of year end moves some of these dates and not others, and shortening a set of accounts can move the notification deadline into the past. What happens to an R&D claim if you change your accounting date works through what a long or short period does to a claim.

When must you tell HMRC you intend to claim?

Within six months of the end of the period of account. The window opens on the first day of the period of account and closes on the last day of the six months following the accounts, so a 30 June year end gives 31 December and a 31 December year end gives 30 June. Getting the exact day right matters: there is no late notification.

The requirement applies to accounting periods beginning on or after 1 April 2023, and catches two groups: companies claiming for the first time, and companies that have not made an R&D claim in the three years ending with the notification deadline. Those three years are measured to the deadline, not to the year end. Our guide to claim notification sets out who is in scope and the narrow exceptions; the notification checker returns your date.

Where the accounts run longer than twelve months, one deadline covers every accounting period inside them. HMRC’s own worked example takes accounts for 1 January 2024 to 30 June 2025, with accounting periods ending 31 December 2024 and 30 June 2025, and gives one notification period running from 1 January 2024 to 31 December 2025. A single form protects both periods.

Miss it and the claim is invalid, however much of the amendment window is left, and there is no appeal.

When is the company tax return due?

Twelve months after the end of the accounting period, in the ordinary case. The legislation takes the latest of several dates, and for a company with straightforward twelve-month accounts that first date — twelve months after the accounting period ends — wins every time. Two others matter where the accounts and the accounting period diverge:

• Accounts of no more than eighteen months. Where the accounts end after the accounting period does, the filing date becomes twelve months from the end of the accounts. A long set of accounts therefore pushes the first return’s filing date out to meet the second’s, and both returns fall due on the same day.
• A late notice to deliver. Three months from the date HMRC served the notice requiring the return, where that beats every other date. This one reaches dormant and newly registered companies brought into the system late.

Corporation tax falls due before the return: usually nine months and one day after the end of the accounting period, or by quarterly instalments for larger companies, which take no deduction for the credit and rise by the tax on it. For a long set of accounts that puts the first period’s tax bill months ahead of the return reporting it.

Filing late does not by itself invalidate an R&D claim, but it moves the enquiry window, and not in the company’s favour.

How long do you have to make or amend the claim?

Two years from the end of the period of account. That is a different clock from the filing date, and it usually runs a year beyond it. Because an R&D claim is made in the company tax return, this is in practice the window for adding relief to a return already filed. How far back you can claim states the rule; backdated claims works through which periods are still open. Accounts drawn up for more than eighteen months run on different rules for both the filing date and the claim window, and need their dates worked out individually.

The Additional Information Form must reach HMRC no later than the claim, and that applies to an amendment as fully as to an original return. Order matters where both go on the same day: the form first, then the return. Where the return arrives first, HMRC removes the claim from it. One form per accounting period, so a long set of accounts needs two. The notification is shared; the forms are not.

HMRC has a discretion to accept a late claim, exercised only in line with a long-standing published statement of practice. Do not plan around it.

What do the dates look like in practice?

Three sets of accounts, worked through. The middle column is HMRC’s own long-period example; the right-hand column is a company that shortened its year end from 31 December to 30 September. Each Additional Information Form goes in no later than its claim, one per accounting period; the enquiry row assumes each return is delivered on its filing date, and the discovery row assumes nothing careless or deliberate.

Each column carries a trap. In the first, the claim window and the enquiry window close on the same day, so a company that files on its filing date and then amends on that last day hands HMRC a fresh enquiry period running to 31 January 2028. In the second, one notification covers both accounting periods and nothing else is shared: two returns, two Additional Information Forms, two claims, and a tax bill on the first period due nine months before the return reporting it. In the third, shortening the year moved the notification deadline from 30 June 2026 to 31 March 2026 — three months earlier, with no change to the underlying work.

How long can HMRC look at the claim?

Twelve months from the day the return is delivered, where the return was delivered on time. Being paid does not shorten it — HMRC paying a claim is not approval of it. Three variants change the date:

• Return delivered late. The window runs to whichever of 31 January, 30 April, 31 July or 31 October first follows the first anniversary of delivery. Filing a day late can hand HMRC up to three extra months.
• Return amended. The same quarter-day rule applies from the first anniversary of the amendment. A claim added by amendment near the end of the two-year window therefore stays open to enquiry well past it — though an enquiry opened on that footing reaches only the amendment and what it affects, which in practice means the claim.
• A company in a group that is not a small group. The twelve months run from the filing date rather than the day the return arrived, so filing early buys no protection.

Once that window closes, HMRC needs a discovery assessment, and it cannot make one at will. It has to establish either that the loss of tax was brought about carelessly or deliberately by the company or someone acting for it, or that an officer could not reasonably have been expected, on the information made available before the window closed, to be aware of the problem. That second limb is the practical argument for disclosing a claim in full: a complete Additional Information Form, computations that tie to the return, and project narratives matching the work actually done make it far harder for HMRC to say afterwards that it could not have known. The outer limits are four years from the end of the accounting period, six where the error was careless, and twenty where it was deliberate. Where something has gone wrong and the return can no longer be amended, voluntary disclosure is the route back; HMRC R&D enquiries covers what happens inside the window.

Which dates are closing now?

The old schemes are on a runway. The merged scheme and ERIS apply to accounting periods beginning on or after 1 April 2024, so every earlier period sits under the old SME scheme or old RDEC, and the last standard old-scheme deadlines fall in late March 2027. Each company’s date is set by its own year end, and for most it falls well before that.

The notification deadlines close more quietly. Where a period ended more than six months ago its notification window has already gone, and if the company was in scope and did not notify, the state of the amendment window is beside the point. A period that ended within the last six months may still be open: count the six months from the day after the accounts end. Settle notification status for each open period before spending time on anything else.

Our deadlines calculator takes your accounting dates and returns every date above; the notification checker covers the one date with no second chance.

Talk it through with a chartered adviser

Deadlines are the part of an R&D claim with no remedy: nothing here can be argued after the date. LimestoneGrey is a firm of Chartered Tax Advisers and Chartered Accountants, regulated by ICAEW, specialising in R&D tax relief. Every claim is signed off by a chartered adviser, and enquiry support is included as standard.

If you want your own dates confirmed against your accounting periods and filing history, get in touch. For the wider picture, start with our R&D tax relief guide.

Sources

• FA 1998 Sch 18 para 14 — the filing date for a company tax return: the latest of twelve months from the end of the accounting period, twelve months from the end of the period of account in which that accounting period ends, and three months from the notice to deliver.
• FA 1998 Sch 18 para 83E — the period within which an R&D claim may be made, amended or withdrawn, and HMRC’s discretion to allow a late claim.
• CIRD81800 — HMRC’s statement of the claim time limit, that claims are made in the return or an amendment to it, and that the late-claim discretion is exercised only under Statement of Practice 5 (2001).
• FA 1998 Sch 18 para 83EA — a claim is invalid unless the additional information has been provided no later than the date the claim is made or amended.
• Additional information you must submit before you claim — one form per accounting period, two where the accounts cover more than twelve months, submitted before or on the same day as the return.
• CTA 2009 s1142A — the claim notification period: from the first day of the period of account to the last day of the six months following it.
• CTA 2009 s1042C and s1045A — the notification requirement for the merged scheme and for ERIS, the three-year prior-claim exemption measured to the last day of the notification period, and the rule that a claim or notification for one accounting period covers another in the same period of account.
• Tell HMRC you want to claim R&D tax relief — HMRC’s worked examples, including the accounts running 1 January 2024 to 30 June 2025 with a notification period ending 31 December 2025.
• SI 2023/813 — the regulations, in force from 8 August 2023, setting what a claim notification and the additional information must contain.
• FA 1998 Sch 18 para 24 — the enquiry window: twelve months from delivery for an on-time return, the quarter-day rule for late returns and for amendments, and the filing-date start for a company in a group other than a small group.
• FA 1998 Sch 18 paras 41 to 46 — the discovery gateway (careless or deliberate conduct, or information not made available to the officer) and the four, six and twenty-year assessment limits.
• Company Tax Returns — the corporation tax payment date, usually nine months and one day after the end of the accounting period.