HMRC R&D Additional Information Form (AIF): how to submit it

What the Additional Information Form (AIF) is and when you need it

The AIF is a mandatory part of the R&D claim process. It gives HMRC structured information about your company, your R&D projects, and the costs you are claiming, before the claim is processed through your Company Tax Return.  

You must submit an AIF for each accounting period you are claiming for. If you are claiming across more than one accounting period, you will need more than one AIF.  

If your period of account is longer than 12 months

A Corporation Tax accounting period cannot be longer than 12 months. If your accounts cover a longer period and you end up filing two CT600s, you will usually need two AIFs if you want to claim for both accounting periods.  

Submission rules that can invalidate a claim

This is the part that trips people up. The rules are strict and HMRC can reject the claim if you get the timing wrong.  

AIF and CT600 timing

• The AIF must be submitted before, or on the same day as, your CT600
• If you submit both on the same day, the AIF must be submitted first

If the CT600 goes in first, HMRC may remove the R&D claim from the return.  

Claim notification is separate and may also be required

Depending on your circumstances, you may also need to submit a Claim Notification Form (CNF) before you claim. You must notify HMRC if you are claiming for the first time, or if your last claim was more than 3 years before the end of the claim notification period.  

The claim notification period ends 6 months after the end of your period of account. To create your own personalised deadline scheduled, use our quick tool.

What you need to prepare before you start

AIF completion is much easier when finance and technical teams agree the project list and cost build-up before anyone goes near the HMRC portal.

Project information

You will need a list of R&D projects being claimed and enough detail to write a short technical summary for the projects you must describe. The AIF rules require different levels of project description depending on how many projects you are claiming for:

• If you are claiming for 1 to 3 projects, you must describe each project.  
• If you are claiming for 4 to 10 projects, you must describe at least 3 projects that cover at least 50% of your qualifying spend.  
• If you are claiming for more than 10 projects, you must describe at least 3 projects covering at least 50% of qualifying spend. If that would require describing more than 10 projects, you describe the 10 highest-spend projects instead.  

For each project you describe, HMRC expects you to cover the scientific or technological basis of the work, including what advance you were trying to achieve, what uncertainties existed, and what you did to resolve them.  

Cost information

You will need your qualifying R&D costs totalled into the relevant HMRC cost categories. HMRC sets out the categories and the rules on what can be included, including treatment of staff, subcontractors, EPWs, software, cloud and data.  

This is also where some common restrictions sit. For example, there are specific rules affecting certain overseas contractor and EPW costs, with limited exceptions.  

Agent details

HMRC asks you to declare details of any agents who were involved in the R&D claim. This means any person or firm acting on your behalf with HMRC. This could be your accountant, tax advisor or R&D tax advisor (like us!) if they helped prepare or submit the claim

Even if you are preparing the claim internally, confirm whether any external adviser supported project scoping, cost modelling, report drafting, or submission.

Step-by-step: how to submit the AIF

Step 1: Confirm which scheme you are claiming under

For accounting periods beginning on or after 1 April 2024, claims are made under the merged scheme or Enhanced R&D Intensive Support (ERIS).

Step 2: Check whether claim notification applies

Do this early. If claim notification is required and you miss the window, you may not be able to claim for that period.  

Step 3: Lock the project list and choose which projects you will describe

Apply the AIF project selection rules and make sure the projects described cover the required proportion of qualifying spend.  

Step 4: Draft the project summaries

Keep them technical and specific. Focus on:

• The advance you were trying to achieve
• The uncertainty and why it was not readily solvable
• What you did to resolve it (tests, trials, iterations, prototyping)

This is what HMRC uses to understand whether you are claiming for genuine R&D.  

Step 5: Finalise the cost breakdown

Make sure your costs map cleanly to the projects and that your apportionments are supportable (for example staff time on R&D versus non-R&D). HMRC provides category guidance and expects claims to follow it.  

Step 6: Submit the AIF and save evidence

Submit the AIF using HMRC’s process and save a copy of what you submitted for your records, including any submission reference information HMRC provides.  

Step 7: File the CT600 in the correct order

File the CT600 after the AIF (or on the same day, but with the AIF submitted first).  

Common mistakes and how to avoid them

Filing CT600 first

This can invalidate the claim. Make the AIF submission part of your CT600 filing checklist.  

Thin project descriptions

Vague descriptions like “improving the product” tend to raise questions. HMRC wants a clear advance, uncertainty, and the technical work done.  

Costs that do not trace back to evidence

If the cost build-up cannot be reconciled to payroll, invoices, and your accounting records, it weakens the claim and can cause delays.

Forgetting extra AIFs for longer periods

If your accounts span more than 12 months and you are claiming across both accounting periods, you may need more than one AIF.  

Leaving it too late

AIF requires technical and finance inputs. Rushing it increases errors and reduces the quality of the narrative.

How far back can you claim?

Most companies think of this as “up to two years”, but the practical limit is driven by Company Tax Return filing and amendment rules, plus the AIF and (where relevant) claim notification requirements. HMRC sets out how to claim through the CT600 and the steps you must take before submitting.  

If you are close to a deadline, check the period end date and work backwards to confirm your window.