How Far Back Can You Claim R&D Tax Credits?

Wondering how far back you can claim R&D tax credits? The headline answer is two years — but a second, harder deadline catches many companies first. Here's exactly how the time limits work, with worked examples.

You can claim R&D tax credits for up to two accounting periods retrospectively — that is, within two years of the end of each accounting period in which the R&D expenditure was incurred. After that window closes, the claim cannot be made and the expenditure is lost.

That two-year limit is the core deadline. But there is a second, harder deadline that catches many first-time or lapsed claimers before they even get there: the Claim Notification Form deadline.

The Claim Notification Form: the deadline most companies miss

Since 2023, companies making their first R&D claim — or whose last claim was submitted more than three years ago — must file a Claim Notification Form (CNF) with HMRC before claiming. The CNF must be submitted within six months of the end of the accounting period to which the claim relates.

Miss the CNF deadline and HMRC will automatically reject your R&D claim for that period, regardless of how well-prepared it is or how clearly the work qualifies. There is no discretion and no appeals route.

For example: if your accounting period ended 31 December 2025 and you are a first-time claimant, the CNF deadline was 30 June 2026. You still have until 31 December 2027 to file the actual claim — but without the CNF, that claim will be invalid.

Because the CNF window (six months) closes long before the claim window (two years), it is the deadline to check first. If you think you may have qualifying R&D in a recent period and have not claimed before, our guide to the Claim Notification Form deadline explains who must file and when.

The two-year submission window

For companies that have already filed a CNF (or are exempt because they claimed within the past three years), the deadline for submitting an R&D claim is two years after the end of the accounting period in question. The claim is made by amending your Company Tax Return for the relevant period.

Using a 31 March year-end as an example:

• Accounting period ending 31 March 2024 → claim deadline: 31 March 2026
• Accounting period ending 31 March 2025 → claim deadline: 31 March 2027

The same two-year window applies to amendments to existing claims — if you have a previous claim that was under-reported, you can amend it within two years of the period end.

What counts as an "accounting period"

For most companies the accounting period matches the 12-month period covered by your statutory accounts and Company Tax Return. Two situations catch people out:

• Long periods of account. A period of account longer than 12 months is split into two accounting periods for Corporation Tax — each has its own two-year R&D deadline and, where relevant, its own CNF deadline.
• Changed year-ends. If you've shortened or extended your year-end, check the exact period dates on your CT600 rather than assuming a calendar year — the deadlines run from the period end, not the calendar year.

Does the merged scheme change the deadlines?

No. The move to the merged R&D scheme (for accounting periods beginning on or after 1 April 2024) changed the rates and rules, but not the time limits. The two-year submission window and the six-month CNF deadline apply just as they did under the old SME and RDEC schemes. What changed is how much your claim is worth, not how long you have to make it.

How to claim for a past period

1. Check the CNF position first. If it's a first claim (or your last claim was over three years ago), confirm the CNF deadline for each period hasn't passed — six months after the period end. 2. Confirm the two-year window is open. Identify the period end and count two years forward. 3. Prepare the technical and cost evidence for each open period, and complete the Additional Information Form — mandatory for every claim. 4. Amend the Company Tax Return for each period to include the claim.

Want a sense of the value before you start? Use our R&D tax credit calculator to estimate a period's benefit, and see our R&D claim checklist for the full submission steps.

The CT600 filing deadline

Your Company Tax Return (CT600) must be filed within 12 months of the end of the accounting period. R&D claims are made within the CT600, so if you have not yet filed your return, the CT600 deadline and the two-year R&D deadline both apply — whichever gives you less time is the binding constraint. In practice, if you file your return on time, the two-year window is the relevant limit for any subsequent amendment.

Frequently Asked Questions

Can I claim R&D tax credits if I've already filed my Corporation Tax Return?

Yes — with one important caveat. You can amend a filed Company Tax Return to improve or correct an existing R&D claim, provided you are still within the two-year window from the end of the relevant accounting period. However, if you have not previously claimed R&D for that period and the Claim Notification Form deadline has already passed (six months after the period end), HMRC will reject any new R&D claim added by amendment — the CNF requirement applies whether you claim in the original return or by later amendment. Where prior claims already exist for the period, amendments to improve them are not affected by the CNF rules.

Does the two-year limit apply if I've never claimed before?

Yes — but you also need to check whether a Claim Notification Form was required for those periods. If you are a first-time claimant and the CNF deadline for a given period has already passed, HMRC will reject the claim for that period even if you are still within the two-year submission window. The CNF deadline is six months after the period end; the claim submission deadline is two years. Both must be met.

Can HMRC extend the two-year deadline?

Extremely rarely. HMRC will only consider an extension in exceptional circumstances — for example, a demonstrable system failure outside the company's control. Workload or advisor error is not accepted as grounds. The practical position is that the two-year window is absolute.

How far back can I claim if I've been R&D-active for several years?

Only the two most recent completed accounting periods are open for new claims at any given time. Earlier periods are time-barred. If you have been conducting qualifying R&D without claiming, acting quickly matters — each period that passes closes permanently. Book a free consultation and we'll assess which periods are still claimable and what the expected value of a retrospective claim would be.

Can I still claim under the merged scheme rules for older periods?

The scheme that applies depends on when the accounting period began, not when you claim. Periods beginning before 1 April 2024 fall under the old SME/RDEC rules; periods beginning on or after that date fall under the merged scheme. The deadlines are the same either way — what differs is the rate and how the relief is given.