For years, an R&D tax relief claim could be a number on a tax return and not much else. HMRC saw the figure. It did not see the projects behind it, the costs split by category, or who had signed it off. If it wanted to know more, it opened an enquiry and asked.
The Additional Information Form (AIF) reversed that order. HMRC now asks the questions first, before accepting the claim. If you are claiming R&D tax relief, the form is the first thing HMRC reads about your work.
What the AIF is
The AIF is an online form that must be submitted before the company’s Corporation Tax return (CT600) containing the claim. It applies to claims for accounting periods starting on or after 1 April 2023. A claim without a valid AIF is not treated as a valid claim. Rates, thresholds and form content change, so treat this as correct as of October 2026 and check HMRC’s current guidance before you file.
In broad terms, the form asks for:
Who is involved. The company, the agent if there is one, and a named person at the company who approves the information.
The claim itself. What type of relief is being claimed and for which period.
The costs. A breakdown by category, such as staff, subcontractors, externally provided workers, software and consumables.
The projects. A description of the R&D, project by project, covering the field of science or technology, the advance sought and the uncertainties faced.
Why it matters more than it looks
The cost breakdown is a spreadsheet exercise. The project descriptions are where claims succeed or struggle.
Before the AIF, a vague technical narrative might never be seen by HMRC unless it was selected for enquiry. Now every claim carries a short written account of the R&D, in a structured format, in front of the same people who decide which claims to look at more closely. Each project is described against the same questions. What was the baseline? What advance were you trying to make? What could not be worked out in advance, and why not?
That structure makes inconsistencies easier to see. If the AIF says one thing, the technical report says another, and the cost lines suggest a third, that mismatch is visible from the outside. None of it is a trap. It simply means the claim has to hold together as a single story.
Where we see AIFs go wrong
Describing the product, not the problem. “We built a new control system” says what you made. HMRC wants to know what you could not have known how to do at the start.
Costs that do not match the narrative. A project described as materials testing, with most of the cost sitting under software, prompts questions.
Too many projects, too thin. The form is built around a small number of main projects. Listing everything dilutes the strongest ones.
Treating it as a form to tick. The wording is short, but each line should be something you could explain to an HMRC officer in a conversation.
Leaving it to the last minute. The AIF has to be in before the return goes in. Leaving it until the deadline week leaves no time to check it against the technical work.
A practical way to approach it
Start with the technical work, not the form. Identify what was genuinely uncertain and who resolved it. Write the project descriptions from the engineering record, such as test logs, design reviews and failed iterations, in plain English. Then build the cost breakdown and read the two together. If an engineer on your team would not recognise the description of their own project, it needs another pass.
It also helps to decide early who the company’s named approver will be. That person confirms the information is accurate, so they should have seen it.
How we handle it
Our team has an engineering background, so we write project descriptions based on conversations with the people who did the work. We extract costs directly from your accountancy software, so you don’t fill in templates, and we check the narrative, the costs, and the AIF against each other before anything is submitted. After submission, we monitor the company’s HMRC tax account, so you hear promptly about repayments or queries.
If you are an accountant working with a client on a claim, we are glad to act as the technical second pair of eyes on the project descriptions while you continue to look after the wider relationship.
Talk to us
If you would like to talk through how an AIF would look for your own projects, there are two easy ways to start.
Book a no-obligation call with John or Jason: www.vantagernd.co.uk/book
Send us a brief overview of your projects for a free Loom video review: www.vantagernd.co.uk/review-your-project
This article is for general information only. R&D tax rules can change, and the requirements for the AIF depend on your individual circumstances, so you should seek professional advice on your specific situation.