Claim notification deadlines: the silent killer of R&D claims

Some R&D claims don’t fail because they lack eligibility.
They fail because the deadline was missed.

Since the introduction of the Claim Notification requirement for accounting periods beginning on or after 1 April 2023, many companies must now tell HMRC in advance that they intend to claim R&D tax relief.

This is done via the Claim Notification Form (CNF).

What makes this dangerous is not the complexity; it’s the timing.

The deadline in simple terms

You can submit a Claim Notification:

• From the first day of the company’s accounting period,
• Up until 6 months after the end of that accounting period.

After that point, if you were required to notify and didn’t, HMRC can refuse the claim entirely.

For example:

• Accounting period: 1 January 2025 – 31 December 2025
• CNF submission window: 1 January 2025 – 30 June 2026

Miss 30 June 2026, and the claim for that period may be blocked.

This operates completely independently from the CT600 filing deadline.

Who needs to submit a CNF?

According to HMRC guidance (see GOV.UK: “Tell HMRC that you’re planning to claim Research and Development (R&D) tax relief”), notification is required if:

• You are claiming R&D tax relief for the first time, or
• You have not made an R&D claim in any of the previous three accounting periods.

There are transitional nuances, so it’s important to review eligibility carefully rather than assume you’re exempt.

HMRC’s guidance makes clear that where notification is required and not made within the statutory window, a claim may not be valid.

Why it’s called the “silent killer”

The CNF does not:

• Require detailed technical reports
• Require cost calculations
• Commit you to making a claim
• Trigger a compliance check

It simply preserves your right to claim.

Yet many businesses miss it because:

• They only recognise R&D activity after year-end
• They assume the CT600 filing covers everything
• They believe they can “decide later”
• They rely on memory rather than process

In reality, once the 6-month window closes, the legislation leaves very little room for discretion.

The claim can fail before it is even assessed technically.

Why filing is low risk

Submitting a CNF:

• Keeps your option open
• Does not increase enquiry risk
• Does not require you to proceed
• Provides compliance protection

Given how often R&D is identified retrospectively, particularly in engineering, software, manufacturing and construction, protecting that option is sensible governance.

HMRC’s position

HMRC’s published guidance confirms:

• Notification must be made within the statutory time limit
• The requirement applies to specific categories of claimants
• The CNF is separate from the Additional Information Form and the CT600

The guidance is clear and process-driven. This is not an area where informal discretion is typically exercised.

Pause for thought

• Do you know whether your company falls within the notification requirement this year?
• Do you have a diary trigger from the start of your accounting period to 6 months post year-end?
• Are you relying on recollection, or a structured compliance process?
• If a qualifying project began unexpectedly mid-year, would you still have time to notify?

The Claim Notification Form is simple.

Missing it is expensive.