Typically six things: detailed technical explanations of each project, evidence of the scientific or technological uncertainties, a breakdown and calculation of the qualifying costs, evidence of staff roles and time allocation, subcontractor agreements, and contemporaneous project records. An enquiry is a demand to evidence the claim, not a verdict on it, and nothing on that list should come as a surprise — it is the material that ought to have existed when the claim was filed.
What each item is testing
The technical explanations are the heart of it. HMRC wants the competent professional’s account: what advance in science or technology the project sought, why existing knowledge did not readily supply the answer, and what was tried. Marketing language about innovation does not survive this stage. Evidence of the uncertainties is the same point in harder form — design iterations, test results, approaches that failed and the reasons they failed.
The cost breakdown has to be a calculation, not a total. Expect to show how the figure was built, how it reconciles to the accounts and the payroll, and why each category qualifies. Staff time draws particular attention: a round percentage applied across a team invites the question of how it was arrived at, whereas an apportionment traceable to timesheets, sprint records or the individuals’ own accounts of their year answers it. Subcontractor agreements matter because the contractual position, not the invoice, determines who is entitled to the relief.
Then contemporaneous records: the ordinary output of doing the work. Project plans, technical notes, code commits, lab books, minutes. Enquiries move fastest when this material existed at submission rather than being reconstructed afterwards, and reconstruction is usually visible.
There is no statutory format for any of it. That cuts both ways — nothing prescribes a particular document, and nothing excuses being unable to evidence what you claimed. Estimates are not fatal either: HMRC accepts that some R&D costs will be an estimated proportion of known expenditure, asks that the estimate is reached by evidence and reason, and expects less formal documentation from a small company than a large one. But an estimate is not a substitute for capturing evidence as the work happens, which is always the cheaper position to be in.
Timing, and what it means for your money
HMRC can withhold payment while it enquires. Where it decides not to pay a claim, its published practice is to aim to open an enquiry within 60 days of receiving the claim — so a payment that has not arrived is not necessarily lost, it may be a claim under examination. Payment does not close the file either. HMRC can enquire into claims it has already paid, which is why receiving the money is best read as processing rather than approval.
None of this is unusual any more. HMRC checked around one in six claims in 2023-24, its latest published figure, and our page on how likely an HMRC enquiry is sets out what sits behind that number. A well-prepared Additional Information Form does much of the groundwork in advance, because it forces the project narrative into the shape HMRC will later test.
Where to go next
Our HMRC enquiries guide walks through the process stage by stage. Enquiry support is included as standard for claims we prepare. If you are facing an enquiry into a claim another adviser filed, we take those on as well, and our free claim review is a confidential way to get an honest read on a claim before HMRC gives you theirs.
Written by Matthew Jones ACA CTA. Last reviewed July 2026.
Sources
- CIRD80525: practice note for ISBC and WMBC — HMRC’s practice on withholding payment and the 60-day aim for opening an enquiry where it decides not to pay.
- GfC3: Recommended approach to claims and record keeping (part 5) — estimates by evidence and reason, and the records HMRC expects.
- Corporation Tax: Research and Development tax relief — what a claim must contain.
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.