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
Taking them in turn:
- 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
- contemporaneous project records
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.
How it runs, and how long it takes
Most enquiries are conducted by correspondence: HMRC writes, you answer, documents follow. But HMRC can ask for a call or a meeting, and it may want to hear from the competent professional directly rather than only from the adviser. Its guidelines for compliance say a check may extend to seeing documents, visiting the site, examining prototypes and final products and talking to employees. A claim built on a real technical account has nothing to lose by that; a claim written for the file does.
Duration depends on what is in dispute rather than on the size of the claim. Where the records exist and answer the questions asked, a check can close in a matter of weeks. Where HMRC contests whether the work met the definition of R&D, or whether a cost head qualifies, months is the realistic expectation and the exchanges can run longer. Nobody can promise you a timetable, HMRC included.
Timing, and what it means for your money
HMRC can withhold payment while it enquires. Its published aim is to pay 85% of payable tax credits within 40 days of receiving the claim, or to make contact about the claim within that time; on its own published figures it processed 92% of claims within 40 days in 2023-24. Those are HMRC’s aims and HMRC’s numbers, not a timetable we or anyone else can commit to on its behalf. 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.
How long that exposure lasts is set by statute. For a return delivered on or before the filing date, HMRC has twelve months from the day the return was delivered to open an enquiry — twelve months from the filing date instead, for a company in a group that is not a small group. Amending the return gives HMRC a fresh window on the amendment, and where the original window has already closed that enquiry is limited to what the amendment changed, which matters because so many R&D claims are made by amendment. Once the enquiry window has gone, HMRC can still make a discovery assessment where relief has been given that is or has become excessive: ordinarily up to four years after the end of the accounting period, six years where the loss of tax was brought about carelessly, and twenty where it was brought about deliberately. Reasonable care keeps you in the four-year band.
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.
Sources
- CIRD80525: practice note for ISBC and WMBC — HMRC’s aim to pay 85% of payable tax credits or make contact within 40 days, its 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, the records HMRC expects, and what a compliance check may extend to: documents, site visits, prototypes and talking to employees.
- FA 1998 Sch 18 para 24 and para 25 — the twelve-month enquiry window running from delivery of the return, the filing-date variant at sub-paragraph (6) for a company in a group other than a small group, the fresh window on an amendment, and the confinement of a late-amendment enquiry to matters the amendment relates to or affects.
- FA 1998 Sch 18 para 41 and para 46 — discovery assessments where relief given is or has become excessive, and the four, six and twenty-year limits by behaviour.
- HMRC’s approach to R&D tax reliefs 2023 to 2024 — 92% of claims processed within 40 days in 2023-24, against the published 85% aim.
- 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.