Five mistakes account for most of the R&D claims that run into trouble: claiming routine work as R&D, inflating costs, filing figures that contradict the accounts, writing vague narratives, and missing the claim notification deadline. Each one is avoidable, and with HMRC checking roughly one in six claims, avoiding them is no longer optional housekeeping.
Most companies claiming R&D relief do so in good faith. These are the places where good faith is not enough.
Are you claiming routine work as R&D?
Challenging and complex does not mean qualifying. To qualify, a project must seek an advance in science or technology, involve scientific or technological uncertainty, and go beyond routine development, testing or updates. The controlling question is the competent professional test: would a capable professional in your field have already known how to solve the problem? The knowledge gap has to exist in the wider field, not just inside your company. Our guide to what counts as qualifying R&D works through the test with sector examples.
Are your costs accurate and apportioned?
Overstated costs are one of the fastest routes to an enquiry. The recurring errors:
- claiming staff who were not directly or indirectly involved in the R&D
- including 100% of a salary where only part of the person’s time went to R&D
- adding marketing, commercial or customer support costs that never qualify
The discipline is boundary-setting: knowing where the R&D project ends inside the wider development project, and keeping time and resource records that support the split. Our page on which costs qualify sets out each category and its rules, including the treatment of subcontractors, where assuming everything qualifies is itself a classic pitfall.
Do your claim figures match your accounts?
A claim does not exist in isolation. If payroll figures in the claim exceed the payroll in your accounts, or subcontractor costs differ from the financial statements, HMRC notices, and inconsistency reads as carelessness at best. Make sure whoever prepares the claim and whoever prepares the accounts are working from the same numbers. A well-documented claim tells one consistent story across the CT600, the accounts and the Additional Information Form.
Does your narrative explain why the work was R&D?
A vague narrative can sink a claim for work that genuinely qualified, because the HMRC caseworker only sees what you filed. A strong narrative covers three things: the technological baseline at the start of the project, the advance sought relative to that baseline, and the uncertainties, why they were hard and how you attacked them. HMRC is not asking what you built. It is asking why building it required R&D rather than the application of existing knowledge. This bites hardest in software claims, where the qualifying work is invisible unless the narrative locates it precisely.
Have you checked the claim notification requirement?
First-time claimants, and companies that have not claimed in the previous three years, must notify HMRC within six months of the end of the period of account. Miss it and no claim can be made for that period at all. The rules, and the traps inside them, are on our claim notification page; our claim notification checker gives you an answer on your own dates in under a minute.
What do these pitfalls have in common?
They are all preparation failures, not eligibility failures. The R&D is usually real; the claim just fails to prove it. That is fixable, and fixing it is considerably cheaper than defending an enquiry after the fact.
If you want a second pair of eyes on a claim before it goes in, talk it through with a chartered adviser.
Written by Matthew Jones ACA CTA. Last reviewed July 2026.
This article describes the rules as they stood at the review date above. The rules change: for the current position, start with our guides or talk to us.
