Is It Too Late to Claim R&D Tax Relief for a Period That Ended 18 Months Ago?

An R&D tax relief claim for a period ending 18 months ago is still within the two-year time limit set by paragraph 83A Schedule 18 Finance Act 1998. Under the merged R&D Expenditure Credit scheme that has applied since April 2024, the time limit to submit or amend an R&D claim is exactly two years from the end of the accounting period. That leaves six months of runway from the 18-month mark. Six months sounds comfortable, but the clock has no snooze button and preparation takes time.

The Two-Year Time Limit: What It Covers

The time limit applies to making, amending, or withdrawing an R&D claim. It is set out in paragraph 83A Schedule 18 FA1998 and applies to the merged RDEC scheme, the Enhanced R&D Intensive Support scheme, and the predecessor SME and RDEC schemes for older periods still within the window.

The calculation is from the last day of the accounting period, not from when the company tax return was filed. For a company with a 31 March year-end:

• Period ending 31 March 2024: deadline is 31 March 2026.
• Period ending 31 March 2023: deadline was 31 March 2025 (now closed).
• Period ending 30 September 2024: deadline is 30 September 2026.

An accounting period ending 18 months before today, which is approximately November 2024 if you are reading this in May 2026, has a deadline of approximately November 2026. That is six months from now. Tight, but achievable if you act now.

The CNF Complication for Post-April 2023 Periods

For accounting periods beginning on or after 1 April 2023, the Claim Notification Form must have been filed by the company tax return deadline to make the claim valid. This is separate from the two-year amendment window.

If your period began on or after 1 April 2023 and you did not file the CNF by the original deadline, the R&D claim for that period is inadmissible regardless of how much of the two-year window remains. The CNF gate is a pre-condition to the claim, not a deadline that runs concurrently with the amendment window.

The critical question for any late claim on a post-April 2023 period is therefore:

1. Was the company exempt from the CNF requirement? (Three-period lookback; see the CNF exemption explained.)
2. If not exempt, was the CNF filed by the company tax return deadline?

If the answer to both questions is no, the period is closed for R&D purposes even if the two-year amendment window is still open. This is the most common reason late claims fail for recent periods.

Amending an Existing Return to Add an R&D Claim

If you already filed a company tax return for the period without including an R&D claim, you can amend the return to add it. The amendment is made through HMRC's Online Services for Corporation Tax. The amendment deadline is the same two-year window.

The amended return must include the R&D claim in the appropriate box (box 650 in the CT600 for RDEC claims) and the revised figures in the tax computation. You must also submit the Additional Information Form for the period. The AIF was mandatory from August 2023, so any claim for a period from that point onwards needs one.

For periods before August 2023, the AIF was not required at the time of original filing. Amendments to those periods do not retrospectively require an AIF unless HMRC specifically requests supporting documentation through a compliance check.

Back-Year Claims: Can You Claim Multiple Periods at Once

Yes. If you have qualifying R&D activity across multiple recent accounting periods and have not yet claimed, you can prepare and submit claims for each open period. Each period has its own two-year window, so they may have different deadlines.

For a company with a 31 March year-end reading this in May 2026:

• Year ending 31 March 2025: window closes 31 March 2027. Two full years remaining.
• Year ending 31 March 2024: window closes 31 March 2026. Less than one year remaining.
• Year ending 31 March 2023: window closed 31 March 2025. Now closed.

If you have qualifying activity in both the 2024 and 2025 periods, you can claim for both, but you need to act on the 2024 period before 31 March 2026. A specialist can prepare both claims simultaneously.

Use our eligibility checker to get a quick read on whether your activities are likely to qualify before committing to the preparation work. Check the qualifying expenditure guide to understand what costs can be included for each period.

What to Prepare for a Late Claim

A claim for a period ending 18 months ago relies on records that were created at the time, not prepared after the fact for the claim. HMRC expects contemporaneous evidence. The further back in time the period, the more important it is that records were kept as part of normal operations.

You will need:

• Payroll records for the qualifying period, broken down by employee to show gross salary, employer NIC, and employer pension contributions.
• Evidence of time allocation for each qualifying employee, such as timesheets, project management records, or sprint logs from version control systems.
• Invoices and contracts for any qualifying subcontractors or externally provided workers.
• Technical documentation describing the projects undertaken, the baseline of knowledge, the technical uncertainties, and the work done to resolve them.
• Software licence invoices or cloud computing billing records for any qualifying expenditure in those categories.

A specialist adviser working on a late claim will ask for all of these at the outset. The quality of the documentation directly affects the quality of the AIF that can be prepared, which in turn affects the likelihood of HMRC accepting the claim without opening a compliance check.