AIF Form: How to Complete HMRC’s R&D Additional Information Form (2026)

by Adam Park | February 28, 2026

The HMRC R&D Additional Information Form (AIF) is a mandatory online submission for UK companies claiming R&D tax relief. Introduced in August 2023, it applies to claims under the SME scheme, RDEC, Merged RDEC (from April 2024), and ERIS. Submitting this form correctly is crucial, as claims without it are automatically rejected.

Key Points:

Submit the AIF before or on the same day as your CT600 tax return.
Claims are rejected if the CT600 is processed first.
You’ll need detailed project descriptions, qualifying costs, and company details.
For hybrid claims (SME + RDEC), include separate project and cost details for each scheme.
Errors like mismatched figures, missing details, or late submissions can invalidate claims.

What You Need:

Company identifiers (UTR, PAYE reference, VAT number, etc.).
Project details: technical challenges, uncertainties, and methods used.
Breakdown of qualifying costs (staff, subcontractors, software, etc.).
For ERIS claims, a 40% R&D intensity threshold must be proven.

Timing, accuracy, and detailed records are critical. Submitting the AIF correctly can help ensure your claim is processed within HMRC’s 40-day window.

HMRC R&D Additional Information Form Submission Process and Requirements

6. How to file compliant research and development tax relief claims

When You Need to Submit the Additional Information Form

Every UK company making an R&D tax relief claim must complete and submit an Additional Information Form (AIF) - no exceptions. This applies whether you're claiming under the SME scheme, RDEC, the Merged RDEC scheme (for periods starting on or after 1 April 2024), or Enhanced R&D Intensive Support (ERIS). Following the submission rules and deadlines is critical to ensure your claim is valid.

For hybrid claims - where you're using both the SME and RDEC schemes in the same accounting period - you’ll need a single AIF. However, you must include separate project details and costs for each scheme. Paying close attention to these requirements can help you avoid mistakes that might jeopardise your claim.

Submission Deadlines and Requirements

The AIF must be submitted via Government Gateway before or at the same time as your CT600 tax return. If the CT600 is processed first, your claim will be automatically rejected.

"If the tax return is submitted before the additional information form, the claim will be rejected." – GOV.UK

Once you start the online draft, you’ll have 28 days to complete and submit it. Be sure to save a copy and note your submission reference number before finalising. After submitting the AIF, tick Box 657 on your CT600 to confirm its submission. Keep in mind that HMRC’s portal does not allow access to the form after submission, so retaining your records is essential.

For accounting periods longer than 12 months, you’ll need to file two CT600s and two corresponding AIFs - one for each period.

Missing the AIF deadline can have serious consequences. When the form was first introduced, around 50% of R&D claims were deemed invalid because the necessary form wasn’t submitted. If your AIF is late or missing, your R&D claim will be removed from the CT600. Worse still, if the amendment deadline has passed, you’ll lose the relief permanently.

Which Claim Types Require the Form

The AIF is a requirement for all R&D tax relief schemes, and the submission rules are consistent across the board:

SME scheme (including claims affected by the PAYE cap introduced in April 2021)
RDEC (Research and Development Expenditure Credit)
Merged RDEC scheme (for accounting periods starting on or after 1 April 2024)
ERIS (Enhanced R&D Intensive Support)

For Northern Ireland companies claiming ERIS, additional declarations are needed regarding de minimis State aid limits and goods trade. If you’re using an authorised agent to submit the form on your behalf via an Agent Services Account, all timing and compliance rules still apply.

Gathering the Required Information

Before diving into your submission, make sure you have all the necessary company identifiers and contact details at hand.

Company Information and Project Overview

Be prepared with key details such as your UTR, PAYE reference, VAT number, SIC code, and the contact information for a senior R&D representative (usually a director). If you're based in Northern Ireland and claiming ERIS, you'll also need your Company Registration Number (CRN), registered business address, and a record of any de minimis State aid received in the past three years.

Double-check that the accounting period dates on your AIF align with those on your CT600. You'll also need to break down your total qualifying expenditure by project, include total relevant costs, and account for any connected company costs if you're claiming enhanced relief.

Once you’ve gathered this information, you’re ready to dive into describing your projects and their technical challenges.

Choosing and Describing Your Projects

The number of projects you need to detail depends on how many you're including in your claim. Here's the breakdown:

For 1 to 3 projects, describe all of them.
For 4 to 10 projects, detail at least 3 that make up 50% or more of your total qualifying expenditure.
For more than 10 projects, describe at least 3 projects covering 50% of the expenditure - or, if it takes more than 10 projects to reach 50%, describe the 10 largest by expenditure.

For each project, include specifics such as the field of work (e.g., 'software/single-page web application development'), the baseline knowledge, the intended advance, the technical uncertainties, and the methods used to address those uncertainties. Avoid general or overly commercial descriptions. HMRC reminds you that "A problem is not an uncertainty if it can be resolved in a discussion with peers". Focus on the complexity of the challenges - what made the solution not easily deducible by a skilled professional? Document the process thoroughly, highlighting failed attempts and the reasoning behind them, not just the final solution.

If you're claiming ERIS, there are additional requirements for documenting R&D intensity.

Documenting R&D Intensity

For Enhanced R&D Intensive Support (ERIS), you'll need to show that at least 40% of your total relevant costs are attributed to R&D activities. Gather your GAAP-based total relevant costs and capitalised R&D expenses, along with the R&D and total costs for any connected companies. Break down qualifying expenditure by categories such as staff, consumables, software, subcontractors, EPWs, clinical trial participants, and - if incurred after 1 April 2023 - cloud computing and data licences. Make sure to also account for the portion of costs associated with Qualifying Indirect Activities (QIAs).

How to Complete Each Section of the Form

Filling out the Additional Information Form (AIF) requires a methodical approach. Use the information you've gathered to answer each section accurately. The AIF is designed to address specific questions, and your responses must be precise, technical, and meet HMRC's standards. The technical and financial documentation previously outlined will be key to ensuring your submission is compliant.

Writing About Technical Uncertainties and Advances

When describing each project, you'll need to address six essential questions: the field of science or technology involved, the baseline knowledge before starting, the advance you aimed for, the uncertainties faced, how you worked to resolve them, and the relief amounts claimed. HMRC focuses solely on scientific or technological progress, so avoid mentioning commercial benefits such as customer acquisition or cost savings unless directly tied to a technical improvement.

When discussing uncertainties, apply the "competent professional" test. HMRC clarifies: "A problem is not an uncertainty if it can be resolved in a discussion with peers". This means you must detail why the challenges weren't straightforward for someone skilled in the field. Be specific and use measurable terms, like "reducing computational time by 30%", rather than vague statements. Highlight the methods, testing cycles, and iterations your team undertook, even if the uncertainty wasn't fully resolved.

Avoid generic or recycled language. Each submission must reflect the unique uncertainties encountered during that specific accounting period. Reusing narratives from previous years is not acceptable. Once your technical descriptions are complete, move on to outlining your financial data.

Breaking Down Your Expenditure

Categorise your qualifying costs according to HMRC's guidelines. These include:

Staff costs (salaries, National Insurance, and pension contributions)
Subcontractors
Externally provided workers (EPWs)
Software, cloud computing, and data licences
Consumables like water, fuel, and power

For each project, provide the total qualifying expenditure. If you're claiming under both the SME and RDEC schemes, ensure costs are separated accordingly.

For accounting periods beginning on or after 1 April 2023, you'll also need to include details about EPWs, such as their employer's PAYE reference numbers. Clearly identify costs related to Qualifying Indirect Activities (QIAs), and if claiming R&D-intensive SME relief, include the total relevant costs, R&D expenditure, and any connected company costs to prove you meet the intensity threshold.

Ensure the R&D figures in your AIF match those in Box 657 of your CT600 and your tax computation. Even minor discrepancies can result in automatic rejection. After categorising your expenditures, you're ready to move on to submitting the form.

Submitting Through Government Gateway

The AIF must be submitted electronically via the Government Gateway. It’s crucial to submit it on or before the same day as your CT600, with the AIF timestamped earlier. If the CT600 is processed first, HMRC will reject your claim and remove it from your tax return. Since the portal doesn't allow access to the form after submission, save a PDF copy for your records immediately.

While the AIF itself doesn't permit attachments, you can include additional technical reports or spreadsheets with your CT600 submission to provide extra context. Accuracy and timely submission are critical for compliance, so double-check everything before finalising.

Common Mistakes and How to Avoid Them

Even with detailed guidance, some common missteps can derail your R&D claim. The Additional Information Form (AIF), introduced by HMRC in August 2023 to reduce fraud and errors, has made the process more stringent. In 2020–21, fraud and errors in R&D claims were estimated to cost £1.13 billion (16.7% of all claims). Since the introduction of the AIF, about 20% of all R&D claims now face compliance checks or enquiries.

Typical Errors and How to Fix Them

One of the most critical mistakes is submitting your CT600 before the AIF. HMRC will automatically reject your R&D claim if the AIF isn’t timestamped earlier - even if both documents are submitted on the same day. Timing is everything here, so make sure the AIF submission takes precedence.

Another frequent issue is financial discrepancies. Even small rounding errors between the AIF and the CT600 can trigger automated rejections. Double-check that every figure in Box 657 of your CT600 perfectly matches the totals in your AIF before you submit.

"If the AIF isn't submitted or is submitted with data missing, HMRC won't consider the claim valid - it will be removed from your tax return."
Linda Eziquiel, R&D Specialist at RandDTax

Project descriptions are another area where claims often falter. Many submissions focus on commercial benefits rather than addressing technical uncertainties. As Millie Palmer, Technical Analyst at Myriad Associates, advises:

"You should be able to describe your advance without mentioning the commercial market".

Avoid phrases like "first to market" or "increased customer satisfaction." Instead, explain the technical challenges that required solutions beyond standard expertise. Palmer also highlights another common pitfall:

"The biggest mistake that always shows up is claimants not answering all the questions equally... It's easy to wax lyrical about your innovation, but HMRC will look just as closely at the uncertainties and your attempts to overcome them".

Administrative errors are equally problematic. Issues like incorrect UTR numbers, mismatched accounting period dates, or wrong SIC codes can lead to claim removal. From 1 April 2024, companies must also provide detailed justifications for costs related to overseas subcontractors and externally provided workers. Ensure all administrative details align with your official records.

Here’s a quick snapshot of how different types of errors can affect HMRC’s response:

Behaviour	Unprompted Disclosure	Prompted Disclosure
Careless	0% – 30%	15% – 30%
Deliberate (not concealed)	20% – 70%	35% – 70%
Deliberate (concealed)	30% – 100%	50% – 100%

HMRC Penalty Rates for Incorrect AIF Submissions

Compliance Checklist

To minimise errors, use this checklist to finalise your submission:

Ensure the correct number of projects are described:
- For 1–3 projects, describe all of them.
- For 4–10 projects, describe at least three, covering 50% of total qualifying expenditure.
- For 11 or more projects, describe at least three, covering 50% of costs, or the top 10 highest-cost projects.
Keep detailed records like timesheets, technical logs, and project notes throughout the R&D process. Don’t rely on memory months later.

If you’re unsure about any part of your submission, consulting specialists like Zest R&D Tax Advisors can be a wise move. They can help flag potential issues before HMRC does, especially for complex claims involving multiple projects or overseas costs. A professional review can make all the difference in ensuring your claim’s success.

Conclusion

Filing HMRC's R&D Additional Information Form (AIF) isn’t just a formality - it’s a critical step in securing your tax relief. To avoid complications, submit the AIF before your CT600, ensure all figures align perfectly, and focus your project descriptions on technical challenges rather than commercial outcomes. These steps are key to preventing automatic claim rejection.

In one striking example from 2025, a software development company missed out on tax savings estimated between £175,300 and £269,700 simply because they failed to submit the AIF before their tax return deadline. Errors can also be costly, with penalties ranging from 0% to 100% of the claim value, depending on their severity. Precision and attention to detail are non-negotiable.

Timeliness is only part of the equation - accurate record-keeping is equally vital. Companies with well-organised records typically spend about two hours completing the form, while those with disorganised documentation can take six hours or more. Keeping timesheets, technical logs, and project notes up to date reduces the risk of errors, especially for complex claims.

For businesses tackling intricate project portfolios or calculating R&D intensity for the first time, professional advice can make all the difference. Zest R&D Tax Advisors specialises in preparing and submitting claims that meet HMRC’s standards, helping companies avoid costly mistakes. Their expertise is especially valuable for first-time claimants or firms managing multiple technical projects.

FAQs

What if I submitted my CT600 before the AIF?

If you filed your CT600 before submitting the Additional Information Form (AIF), your R&D claim could be at risk. HMRC mandates that the AIF must be submitted either before or at the same time as the CT600. Missing this requirement could lead to your claim being rejected. To prevent delays or issues, make sure the AIF is submitted promptly alongside your CT600.

How do I split projects and costs in a hybrid (SME + RDEC) claim?

To divide projects and costs in a hybrid claim involving SME and RDEC schemes, you need to categorise projects based on which scheme they qualify for and allocate costs to the appropriate category. Ensure you clearly outline project details, associated costs, and the applicable scheme in HMRC’s Additional Information Form (AIF). Accurate documentation, such as timesheets and invoices, is essential to back up your submission and meet HMRC's compliance standards.

What evidence do I need to prove the 40% ERIS R&D intensity?

To demonstrate the 40% ERIS R&D intensity, you need to ensure that your eligible R&D expenditure meets the specified threshold. Here's a breakdown of what you should focus on:

Staff Costs: Include salaries, pensions, and National Insurance contributions for employees directly involved in R&D activities. Ensure you can document their roles and time spent on eligible projects.
Subcontractor Expenses: Costs associated with subcontractors engaged in R&D work can qualify, provided their contributions are directly linked to the projects addressing scientific or technical uncertainties.
Consumables: Include materials and utilities consumed during the R&D process, such as prototypes, testing materials, or lab supplies. Keep detailed records of usage.
Software Costs: If software is acquired or developed for R&D purposes, ensure these expenses are clearly documented and tied to the projects.

Support these costs with technical and financial documentation, such as invoices, payroll records, contracts, and time logs. Additionally, provide project descriptions that clearly outline the scientific or technical uncertainties tackled, demonstrating how they align with the appropriate SIC codes and R&D intensity requirements. Maintain thorough reports and records to show compliance with the criteria.