Generally two years from the end of the period of account. For accounting periods beginning on or after 1 April 2023, the R&D time limit is its own rule: a claim must be made, amended or withdrawn within two years of the last day of the period of account the claim relates to, or within 42 months of its first day where the period of account is longer than 18 months. For most companies, whose accounts and accounting period are the same twelve months, that means what people expect: at any moment you can usually still claim for your two most recently ended years. But the clock runs from the period of account, so where a long period of account contains two accounting periods, both share one deadline and the earlier one gets materially less than two years from its own end.
The 2027 line — and the deadlines already gone
The two-year window is why the old SME and RDEC schemes are still live. They apply to accounting periods that began before 1 April 2024, and for the last of those periods, with a standard twelve-month period of account, the claim window runs out in late March 2027 (a long period of account can push the true long-stop into the autumn of 2027). Just as important is the other direction: the deadlines roll shut year end by year end, and many have already passed — a company with a 31 March 2024 year end lost its last old-scheme period in March 2026. A company that never claimed, or under-claimed, for a still-open year has a runway, and it is shortening on a fixed schedule; when it closes, the old schemes close for good, and with them the loss-making SME rates of up to 33.35p per £1 that applied to expenditure before April 2023. Work the deadline out from your own year end rather than from a headline date.
The trap that closes claims early
The claim window is not the only gate. For accounting periods beginning on or after 1 April 2023, a company claiming for the first time, or that has not claimed in the three years ending with the notification deadline, must have sent HMRC a claim notification within six months of the end of the period of account. Miss that window and the claim is invalid even though the two-year window is still open; there is no late route. One wrinkle is especially unforgiving: a claim that appears in a return only because of an amendment made on or after 1 April 2023, for a period that began before that date, does not count as a prior claim for this test — so a company that thinks of itself as an established claimant can be caught as if it were new. In practice the notification rule closes more backdated claims than the deadline itself does.
Two further cautions. Extended or shortened periods of account move every date on this page — a shortened period pulls the notification deadline forward — so check your own periods rather than assuming twelve-month years. And a backdated claim made now still needs its own Additional Information Form, mandatory for every claim made since 8 August 2023.
Where to go next
Our backdated claims guide works through the remaining runway year by year, and the claim notification deadline checker tests your own dates in a minute. If an older period looks claimable, move before the window decides for you.
Written by Matthew Jones ACA CTA. Last reviewed July 2026.
Sources
- FA 1998 Sch 18 para 83E — the R&D claim time limit: two years from the last day of the period of account, or 42 months from its first day for long periods.
- CIRD81800: time limits for claims — HMRC’s guidance on the limits, including the position for earlier periods.
- Tell HMRC you plan to claim — the six-month notification window and who it catches.
This page describes the rules as they stood at the review date above, as general information rather than advice on your circumstances. For how that distinction works, see our terms; for an answer on your own facts, talk to us.