Tools

R&D deadline calculator

An R&D claim is governed by six separate clocks, and they do not run together. The return has a filing date. The claim has its own two-year limit. A first-time claimant must notify HMRC, or make the claim itself, by the notification deadline, and missing that ends the claim whatever the other dates say. The Additional Information Form has no date of its own but a strict sequence. HMRC's enquiry window opens off the day the return arrives, not the year end. And behind all of it sit the discovery long-stops, which outlive every other date by years.

Enter one period of account and this works out all of them. Each date comes with the rule behind it and a link to the guide that derives it, so you can check the reasoning rather than take the number on trust.

Why the dates diverge

The four windows this calculator dates, and what each one runs from.
The clock	The window
Filing date for the return	12 months the later of twelve months from the end of the accounting period and twelve months from the end of the period of account (FA 1998 Sch 18 para 14(1))
R&D claim or amendment deadline	two years from the end of the period of account, for accounting periods beginning on or after 1 April 2023 (FA 1998 Sch 18 para 83E)
Claim notification deadline	six months after the period of account ends, unless the company made an R&D claim in the three years ending with that deadline (CTA 2009 s 1142A)
HMRC's enquiry window	12 months from the day the return was delivered, where it was delivered on or before the filing date (FA 1998 Sch 18 para 24(2))

Table notes

The Additional Information Form has no window of its own. It must reach HMRC no later than the day the claim is made or amended, and a claim made without it is invalid (FA 1998 Sch 18 para 83EA).
Behind all four sit the discovery long-stops: four years from the end of the accounting period ordinarily, six where the loss of tax was brought about carelessly, and twenty in the cases FA 1998 Sch 18 para 46(2A) covers. They are outer limits rather than a schedule.

Whether the work qualifies, what a claim is worth and whether you clear the 30% intensity threshold are separate questions, answered by our other R&D calculators.

Deadlines are the part of an R&D claim with no remedy attached. If one of yours looks close, or looks passed, that is a conversation to have this week.

End date of your period of account Start date of your period of account

We have assumed a 12-month period of account — change this date if yours is different.

Has the company claimed R&D relief before? No, or not sure The checker will assume a claim notification is needed. Yes Tell us when the last claim reached HMRC and we will apply the three-year test.

Date the return was, or will be, delivered (optional)

Leave blank if you do not know it yet. Without it we can state the enquiry rule but not the date, because the window runs from the day the return arrives.

Rates and rules last reviewed 29 September 2026.

Illustration only

This calculator is provided for illustration only. It applies the standard rules to the dates you enter, but real positions turn on details it cannot see: the statutory definition of the period of account, whether the notice to deliver was served late, whether the company is in a group other than a small group, the exceptions to the three-year notification look-back, and HMRC's limited easements and discretions. Every date it gives is the last day of a window rather than a date to work to. Its output is not tax advice and must not be relied on for any decision, filing or deadline. We build and check these tools carefully, but they are provided as they are, without warranty that they are accurate, complete or current: the rules change, and the tool may itself be wrong. To the fullest extent permitted by law, we accept no liability for any error in it, or for any loss arising from its use or from reliance on its output. Our full website terms apply. Before acting — especially where a deadline looks close or passed — have the position confirmed by a competent chartered tax adviser. We will check it properly.

The rules applied, and where they come from

Every date this tool produces comes from one of the rules below. Each links to the primary source, so nothing here has to be taken on trust. Two things it is not: it is general information rather than advice, and it is not a substitute for checking a deadline that matters against your own facts.

A period of account longer than 12 months divides into a 12-month accounting period and a stub, because an accounting period ends at the latest twelve months from its beginning.

CTA 2009 s 10(1)
The filing date is the last of the dates para 14 gives: twelve months from the end of the accounting period, or twelve months from the end of the period of account.

FA 1998 Sch 18 para 14(1)
A company may not amend its return more than twelve months after the filing date.

FA 1998 Sch 18 para 15(4)
An R&D claim must be made, amended or withdrawn within two years from the end of the period of account. Accounting periods beginning before 1 April 2023 instead run to the first anniversary of the filing date for that period’s own return.

FA 1998 Sch 18 para 83E
The claim notification period ends with the last day of the period of six months beginning with the first day after the period of account, and the requirement catches accounting periods beginning on or after 1 April 2023. A company is outside it where it made an R&D claim in the three years ending with that last day. A company it catches must either notify HMRC by that day or make the claim itself by then.

CTA 2009 s 1142A and s 1042C
A claim is invalid unless the Additional Information Form was provided no later than the day the claim was made or amended, and the form goes in at least once for each accounting period claimed for. It applies to claims made on or after 1 August 2023 — in practice 8 August 2023, when the regulations came into force.

FA 1998 Sch 18 para 83EA and SI 2023/813
HMRC may give notice of enquiry up to twelve months from the day a return was delivered where it was delivered on or before the filing date; where it was delivered late, or where the company amends it, the window runs to the quarter day next following the first anniversary of that delivery or amendment. The quarter days are 31 January, 30 April, 31 July and 31 October. An enquiry opened off an amendment once the ordinary window has shut is limited to what the amendment changed.

FA 1998 Sch 18 paras 24 and 25(2)
An assessment may not ordinarily be made more than four years after the end of the accounting period, six years where the loss of tax was brought about carelessly, or twenty where it was brought about deliberately or where the company failed to notify its chargeability or to meet a disclosure obligation for a notifiable arrangement — and only where the discovery gateway is open at all.

FA 1998 Sch 18 paras 41, 43, 44 and 46
The merged R&D expenditure credit and ERIS apply to accounting periods beginning on or after 1 April 2024; earlier periods sit under the old SME scheme or old RDEC.

FA 2024 Sch 1 para 16 and SI 2024/286

What it does not do

Para 14(1)(d) is not applied: where HMRC served the notice to deliver late, the filing date can be three months from the date of that notice, which no calculator can know.
The enquiry window assumes the company is not a member of a group other than a small group. For a company that is, para 24(6) runs the twelve months from the filing date rather than from the day the return was delivered.
The discovery long-stops are outer limits, not a schedule. A discovery assessment can only be made where the gateway is open at all: careless or deliberate behaviour (FA 1998 Sch 18 para 43), or an officer who could not reasonably have been expected to be aware of the situation from the information made available (para 44).
Bank holidays and weekends do not move any of these dates; none of them is a "working day" rule.
Windows can be reopened or extended in ways no calculator models: an officer may allow a late R&D claim under para 83E(5), and HMRC operates a narrow administrative easement for notification periods that ended between 8 September and 30 November 2024.
No delivery date was entered, so the enquiry window is stated as a rule rather than a date.
No prior claim was entered, so the notification answer assumes the company has not claimed within the three years ending with the deadline.

Behind the dates

One period of account worked through date by date, and the questions those dates raise most often.

A worked example

A period of account running 1 April 2025 to 31 March 2026, with the return delivered on its filing date.

The filing date is 31 March 2027, and the R&D claim or amendment deadline 31 March 2028. The claim notification deadline is 30 September 2026: eighteen months before the claim deadline and the first date the period produces. The Additional Information Form has no date of its own and must reach HMRC no later than the day the claim is made. The enquiry window closes on 31 March 2028, and the discovery long-stops run to 31 March 2030, 31 March 2032 and 31 March 2046.

One row is worth pausing on. Amend the return on the last day of the claim window and the enquiry window reopens, running to 30 April 2029: thirteen months past the date the ordinary window closes. Every date here is the last day of a window rather than a date to work to, and a claim filed at the last available moment is not a claim filed safely.

Period of account 1 April 2025 to 31 March 2026

Every date a period of account running 1 April 2025 to 31 March 2026 produces, with the return delivered on its filing date.
Claim notification deadline	30 September 2026
Filing date	31 March 2027
R&D claim or amendment deadline	31 March 2028
Enquiry window closes	31 March 2028
Discovery long-stop, ordinary	31 March 2030
Discovery long-stop, careless	31 March 2032
Discovery long-stop, deliberate	31 March 2046
Enquiry window if you amend on the last day	30 April 2029

Because they are anchored to different things. The filing date and the claim deadline run from the period of account; the discovery limits run from the accounting period; the enquiry window runs from the day the return was delivered. For a company with a straightforward twelve-month year those distinctions stay invisible. Change the year end, or draw up accounts for longer than twelve months, and they separate — a period of account over twelve months is not one accounting period at all but two, each with its own return, its own claim and its own form.

Questions

How long is the R&D claim window?

Two years from the end of the period of account, for accounting periods beginning on or after 1 April 2023 (FA 1998 Sch 18 para 83E). A period of account ending 31 March 2026 runs out on 31 March 2028. Accounting periods beginning before 1 April 2023 take a different anchor entirely, running to the first anniversary of the filing date for that period's own return, and how far back you can claim sets both out.

One date is worth singling out. The claim notification deadline is unforgiving. It falls on the last day of the six months beginning with the day after the period of account ends. There is no late route and no appeal: a claim made after it without a notification is invalid even where the two-year window is still open.

Where a period began before 1 April 2024 it sits under the old SME scheme or old RDEC rather than the merged scheme and ERIS, and the last standard old-scheme deadlines fall in late March 2027. Which regime applies turns on your own accounting dates, covered in which scheme applies to your company.

When does the Additional Information Form have to be submitted?

No later than the day the claim is made or amended, so before or with the return that carries it, and at least once for each accounting period claimed for (FA 1998 Sch 18 para 83EA; CIRD182000). A claim made without it is invalid. The requirement applies to claims made on or after 1 August 2023 — in practice 8 August 2023, when the regulations came into force.

When does HMRC's enquiry window close?

Twelve months from the day the return was delivered, where it was delivered on or before the filing date (FA 1998 Sch 18 para 24(2)). A return delivered late runs instead to the quarter day next following the first anniversary of delivery, which can hand HMRC up to three further months. The window runs from delivery rather than from the year end, which is why the calculator asks for that date and states the rule rather than a date when it is not given.

Does amending the return reopen the enquiry window?

Yes. An amended return can be enquired into up to the quarter day next following the first anniversary of the amendment (FA 1998 Sch 18 para 24(4)), and an enquiry opened on that footing once the ordinary window has closed reaches only the amendment and what it affects. Most R&D claims are made by amendment, so this is the ordinary case rather than an edge one.

How far back can HMRC go under discovery?

Four years from the end of the accounting period ordinarily, six where the loss of tax was brought about carelessly, and twenty under FA 1998 Sch 18 para 46(2A), which is not the deliberate case alone: it also runs where the loss of tax is attributable to a failure to notify chargeability or to a disclosure failure for notifiable arrangements. These are outer limits rather than a schedule, and an assessment can only be made where the discovery gateway is open at all.