The Most Common Mistakes That Get R&D Tax Relief Claims Rejected

R&D tax relief claims fail in two ways. HMRC can remove a claim from the Company Tax Return because a form was missing or late, without anyone reading the technical case. Or it can open a compliance check, an R&D tax claim investigation into whether the work and the costs qualify. A first-time claimant can lose the relief either way, and the first kind of failure is decided months before the return is filed.

This guide sets out the mistakes behind both, using HMRC's own figures and guidance, so a company can check its draft claim against them before it goes in. For the wider picture of how the relief works, see the guide to r&d tax credits.

How Often HMRC Adjusts or Refuses a Claim

HMRC's R&D tax credits statistics for 2023 to 2024 show 61,000 claims received and compliance checks opened on 9,700 of them, 17% of the total (HMRC, "Approach to Research and Development tax reliefs 2023 to 2024", published 30 October 2024). Of the claims it checked, 77% needed an adjustment, up from 71% the year before.

The same report puts error and fraud across the R&D reliefs at 7.8% of relief paid in 2023 to 2024, or £0.6 billion, and records £441 million identified as incorrectly claimed through compliance checks that year.

What a Refused Claim Costs the Company

For accounting periods beginning on or after 1 April 2024, most companies claim under the merged scheme, which gives an expenditure credit of 20% of qualifying spend (GOV.UK, merged scheme and enhanced R&D intensive support guidance, published 18 March 2024, last updated 8 January 2026). A loss-making SME whose relevant R&D expenditure is at least 30% of its total expenditure can instead use enhanced R&D intensive support: an extra 86% deduction and a payable credit worth up to 14.5% of the surrenderable loss.

A refusal removes relief at those R&D tax relief rates. If the relief has already been paid, HMRC recovers it, and a company that was careless can also face a penalty.

Mistake 1: Treating Commercial Development as R&D

The R&D tax credit challenges HMRC raises most often concern the work itself. Relief is refused where the project was routine development, where the technology was new to the company but not to its field, or where the company adopted an existing solution rather than advancing one. Francis Wilks & Jones, a firm that handles disputed claims, calls the last of these a "catch-up" claim (last updated 23 April 2026).

The test turns on a scientific or technological uncertainty that a competent professional in the field could not readily resolve. A claim has to show who that professional is and what they judged uncertain. Where the boundary of a project sits decides which activity can be claimed, and the guide to where an R&D project starts and stops covers that boundary in detail. A company unsure whether its work passes at all should start with the qualifying test in plain English.

Mistake 2: Missing a Deadline That Voids the Claim

Two administrative deadlines invalidate a claim regardless of how strong the work is. An R&D tax claim deadline missed here cannot be fixed by better evidence later.

The Claim Notification Form

For accounting periods beginning on or after 1 April 2023, a company claiming for the first time, or whose last claim was more than three years before the end of the notification period, must send HMRC a claim notification form. The window closes six months after the end of the period of account, and GOV.UK states that without it "your R&D tax relief claim will be invalid" (GOV.UK, "Tell HMRC that you're planning to claim Research and Development (R&D) tax relief", last updated 31 October 2024).

The window runs from the start of the period of account, so a company can notify early. A first claim prepared after the window has closed cannot be made for that period.

The Additional Information Form

The additional information form must reach HMRC before or on the same day as the Company Tax Return, or HMRC removes the R&D claim from the return (GOV.UK, "Submit detailed information before you claim Research and Development (R&D) tax relief", last updated 26 September 2025). The form describes the projects: all of them where there are one to three, or enough projects to cover at least half the qualifying expenditure where there are more.

A form that names the technology without describing the uncertainty does little to support the claim. What a weak technical write-up looks like to HMRC is covered in its own guide.

Mistake 3: Claiming Under the Wrong Scheme

A claim prepared to the pre-2024 rules is prepared to rules that no longer apply. For accounting periods beginning on or after 1 April 2024, the choice is between the merged scheme expenditure credit and enhanced R&D intensive support, and the intensity test (relevant R&D expenditure of at least 30% of total expenditure) decides which one a loss-making SME can use (GOV.UK merged scheme and ERIS guidance, last updated 8 January 2026).

Getting the scheme wrong changes the value of the claim and can turn an accurate cost schedule into an inaccurate claim. Mistakes run in the other direction too: understating a claim is common, and the guide to qualifying costs companies routinely leave out covers the costs a first claim tends to miss.

Mistake 4: A Claim Nobody in the Company Can Defend

A claim is the company's, even when an adviser prepares it. If HMRC opens a check, someone in the company has to explain the projects, the uncertainty and the costs. A claim that only the adviser understands is hard to defend, and the warning signs of an adviser overclaiming are set out in a separate guide.

Contemporaneous records carry more weight in a compliance check: notes made while the work happened count for more than a narrative written a year later. A first claim rarely has a full documentation trail, so the practical step is to gather what exists: project plans, test results, emails recording problems and changes of approach, and time records for the staff involved.

Does Advance Assurance Protect a First Claim?

Advance assurance is available to SMEs, and full claim advance assurance covers the first three accounting periods only (GOV.UK, "Research and Development tax relief advance assurance", last updated 18 May 2026). GOV.UK states that "asking for advance assurance is not the same as making an R&D claim".

Holding R&D tax credits advance assurance does not make a later claim correct, and not holding it is not evidence of anything. The claim notification form and the additional information form are still required.

Check These Before the Return Goes In

This R&D tax credit checklist covers the failures above in filing order:

• Notification: is the company inside the claim notification window, and has the form been sent?
• Projects: does each project describe a specific scientific or technological uncertainty, and who judged it?
• Additional information form: does it cover the right projects, and will it reach HMRC before or with the return?
• Scheme: which scheme applies to this accounting period, and does the intensity test change it?
• Costs: is every cost category evidenced, and has anything been left out?
• Defence: could someone in the company explain the claim without the adviser present?
• Time limit: is the claim within two years of the end of the period of account, where that period is not longer than 18 months (HMRC manual CIRD81800, last updated 7 August 2026)?

HMRC publishes its own list of recurring errors at CIRD80540, although its entries are framed around the old SME and large-company schemes, and its Guidelines for Compliance GfC3 sets out how to check whether work qualifies (published 31 October 2023, last updated 23 January 2025).

Next Step: Size the Claim Before You File

Run the R&D claim estimator to see what a claim is likely to be worth before committing time to the forms, and check the notification window for the accounting period at the same time.