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The Additional Information Form is a crucial part of a R&D tax claim. Submitted before the CT600, it explains why you qualify and how much you're claiming.
Technical Analyst/Writer
Published on: 19/03/2025
Last updated on: 12/11/2026
10 minute read
R&D tax credits are a vital lifeline for many innovative companies in the UK, offering up to 27% of eligible R&D expenditure back in a tax saving or—sometimes—a cash credit.
To submit your claim, you need to fill out the right boxes in your Company Tax Return, but before you send it off, you also need to fill in and submit your Additional Information Form. This form is HMRC’s first line of defence against fraud and error and it’s an essential document to defend your claim in the event of an audit.
The Additional Information Form (AIF) is an online form that all claimants need to submit before their claim is made. It clarifies what you’re claiming for and why you think it’s qualifying.
It was introduced in August 2023 to reduce error and fraud in the scheme. Companies must provide project and expenditure details when making their claim to check if the claim qualifies at a basic level.
The AIF also requires the contact details of the person submitting the claim through an authorised portal, cracking down on disreputable agents and individuals making claims without approval.
The Autumn Budget 2026 confirmed that the Additional Information Form (AIF) will stay in place, but the government intends to simplify it from autumn 2027. HMRC hasn’t yet published what will change, so everything on this page still applies to the claims you’re making now. We’ll update this guide once the detail is confirmed. Read our summary of the Autumn Budget 2026 for the full R&D announcements.
The results of HMRC’s compliance measures are already showing up in HMRC’s statistics. Claim volume is down by 52% compared to the peak of the number of claims in the 2020-21 period. SME claims under any form of the R&D tax credit scheme are reducing year-on-year. This suggests that ineligible claims are being weeded out, which has given HMRC the confidence to reduce some of their compliance efforts.
All R&D tax credit claims made since August 2023 must have an AIF.
There is no exception to this rule; SMEs, large companies, first-time claimants and repeat claimants must all submit an AIF.
Unlike the claim notification form, there is no specific deadline for submitting the AIF. The only requirement is that it is submitted before the Company Tax Return (CT600).
The deadline to make a claim for R&D tax is two years after the end of the accounting period. If you submit your CT600 on the last day, you need to make sure you submit your AIF first.
If you don’t submit the AIF first, the R&D tax claim will not be accepted and you’ll need to resubmit your AIF and then your CT600.
As a representative of the company, you’ll need the Government Gateway user ID and password you used when you registered for Corporation Tax. You can submit using HMRC’s portal for the AIF.
Only representatives of the company or registered agents can submit an AIF for a company.
You must make a separate R&D tax claim for each accounting period, so it makes sense that each accounting period should have its own AIF.
For companies with extended accounting periods, you’ll need to submit two AIFs; one for the first 12 months and another for the following short accounting period.
HMRC needs to know some basic information about your company:
To ensure a third party can’t make a claim on behalf of your business without your knowledge, you need to provide the contact details of the individual responsible for the claim (usually a company director or the finance lead).
Any third-party agent involved in preparing or submitting the claim should also be named. This includes specialist R&D tax advisers (like us!), as well as any accountants who have helped prepare the claim.
You’ll need to provide a summary of the expenditure for each scheme you’re claiming under.
In most cases, you’ll only be claiming under one scheme. However, if you are claiming some projects under the ERIS scheme and some under the merged RDEC scheme, you’ll need to separate those out.
The AIF requests totals for the expenditure in each cost category claimed and a total for the qualifying direct costs versus the qualifying indirect costs. You’ll also need to separate out the total costs per project later in the AIF.
You’ll also have to include details of any connected companies that were connected within the accounting period (even if it was only for a day!).
Companies claiming under the Enhanced R&D Intensive Support (ERIS) scheme need to meet a R&D intensity condition. To prove that you meet the R&D intensity condition, you’ll need to use the AIF to provide:
For each project that you need to write, you’ll need to describe some basic information about the R&D work you did.
Your R&D project must be in a field of science or technology. You should specify further than this; for example, “mechanical engineering”, “information technology” or “pharmaceutical development”.
Since a key qualification criterion of the R&D tax scheme is having an advance in the field, you need to explain what the field looked like before so HMRC has something to compare your advance to. You’ll have to describe the scientific or technological baseline, including the existing state of the art and the knowledge and capabilities in the field at the outset of your project.
An advance is crucial to your R&D project being eligible. You need to show how you sought to improve upon the state of the art in the wider field. You may be able to quantify your advance, or it may be intangible.
You must include the scientific and technological uncertainties you faced when trying to achieve your advance. It needs to be clear why your expert team could not easily work out how to achieve the advance. Steer well clear of mentioning commercial difficulties; HMRC is only interested in your science and technology.
This section will be a brief outline of the R&D activities you carried out in the period. A reviewer at HMRC should be clear on why the R&D project was not straightforward and whether the uncertainties were resolved. If they weren’t resolved, it should be clear why not.
You also need to provide a total expenditure per project.
The amount of text needed in the project descriptions depends on the project and the question. However, you’ll want to avoid writing too much or providing too much field-specific detail. If HMRC can’t understand your projects, they may ask for clarification, which is just doubling your work. Stick to layman’s terms, where possible.
Using a specialist R&D tax adviser can avoid these tricky questions, as they will have a better idea of how much detail is appropriate and will keep to it when writing up your projects. Myriad’s online guided portal, Tax Cloud, has character limits to keep you in reasonable margins.
HMRC has specific guidelines for how many R&D projects need to be written up:
For claims which are made across both the ERIS scheme (which is treated as tax relief) and the merged RDEC scheme (which is an expenditure credit), the above rules apply separately to the schemes. You’ll need to assess the number of tax relief projects you have and the number of expenditure credit projects you have, then apply the above rules to those totals individually.
For example, if you have 8 projects under ERIS and 2 RDEC projects, you’ll need to provide project details for:
This is not a requirement, but you’ll find most specialist R&D tax agents will write up and submit an additional claim report. This report will include more detailed cost breakdowns and will pre-emptively answer some questions on project qualifications and methodologies which may be asked in an R&D tax compliance check.
It's much easier to handle this while preparing your claim rather than months later during a compliance check. Given HMRC's heightened scrutiny to reduce errors and fraud in the scheme, it's wise to plan for the worst-case scenario upfront by organising your project details early rather than rushing at the last minute.
You can submit supplementary information via email or when you submit your CT600.
At Myriad, we support your claim at all stages, from project identification to submission to HMRC. Following a discussion on your projects, we will write up a full report on your R&D and prepare a full costing breakdown. We’ll even submit the AIF on your behalf as a registered agent.
Get in touch or call us on 0207 118 6045 to learn how we can assist you with your R&D tax credit claims.
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