The R&D claim itself is usually the easy part — founders who have genuinely built something technically uncertain can normally tell that story. The part that quietly kills claims now has nothing to do with the science. It is a deadline that falls, for most companies, before the accounts even exist: the claim notification form.

Miss it, and HMRC does not ask any questions about whether your R&D qualified. It strikes the claim out of your Company Tax Return automatically. There is no appeal on the merits, because the rule was never about the merits.

What the claim notification form actually is

For accounting periods starting on or after 1 April 2023, a company must tell HMRC — in advance, using a separate online form — that it intends to make an R&D claim, if either of these applies:

  • It has never claimed R&D relief before, in any accounting period.
  • Its most recent claim was made more than three years before the last day of this period's notification window.

If neither applies — you claimed for the immediately preceding period, or for a period within the last three years — you are exempt and can simply claim on the return as before. The form exists to catch two groups: startups claiming for the first time, and companies that claimed once, went quiet, and came back to it later than they realised.

The deadline: six months, not twelve

This is the trap. The Company Tax Return itself is not due until 12 months after the end of the accounting period. The claim notification form is due 6 months after the end of the accounting period — half the time, and long before most companies have even started preparing their accounts.

Timeline from a 31 March year end: notification form due 30 September, six months before the tax return and R&D claim itself are due the following 31 March Year end 31 March 2026 — the two deadlines Illustrative dates for a company with a 31 March accounting period. 31 Mar 2026 Period ends 30 Sep 2026 Notification deadline (6 months) 31 Mar 2027 CT600 + AIF + claim due (12 months) Most founders start thinking about the R&D claim when the accounts are prepared — by then the notification window has been shut for months.

Submit the return with an R&D claim on it after the notification window has closed, having not notified, and HMRC removes the claim from the return. It does not matter whether the spend was real, whether it was technically uncertain, or whether your accountant simply had not been engaged yet. The form is procedural, and procedural deadlines do not bend.

Worked example: £27,000 lost to a form nobody knew existed

The figures below are illustrative. A SaaS company has a 31 March 2026 year end. It has been trading since 2022 but has never claimed R&D relief — the founders assumed it was for labs and universities, not a product team shipping features. Over the year to 31 March 2026 it spent £180,000 on qualifying staff costs, subcontracted development and software for genuinely uncertain technical work.

Because this would be a first-ever claim, notification was required by 30 September 2026. The founders do not bring it up with their accountant until November, while gathering paperwork for the January board meeting — six weeks after the window closed.

What was lost. Under the merged R&D expenditure credit scheme, £180,000 of qualifying spend attracts a 20% credit: £36,000. That credit is itself taxable, so at the 25% main rate of corporation tax the net benefit is £36,000 − £9,000 = £27,000. None of it is claimable. The R&D was real and would have qualified. The claim fails purely because nobody submitted a form within six months of a year end that, in September, felt a long way from tax return season.

It is not the same as the Additional Information Form

Founders who have heard of "the R&D form" often mean the Additional Information Form (AIF), and conflate the two. They are different forms with different jobs. The AIF is required for every R&D claim, first-time or not, and sets out the detail behind the claim — the qualifying costs, the projects, the uncertainties addressed. It must be filed before, or on the same day as, the Company Tax Return that contains the claim.

The claim notification form is narrower and earlier: a short heads-up, filed only by first-time or lapsed claimants, due six months after the period end — often before there is enough detail to complete an AIF properly at all. You can miss either one and lose the claim. They have to both be done, and the notification has to be done first.

What the form actually asks for

It is a short online submission through HMRC's digital service, not a technical narrative. You will need:

  • The company's Unique Taxpayer Reference and the accounting period being claimed for.
  • The main senior internal R&D contact, plus any agent involved in preparing the claim.
  • A brief outline of the planned claim — enough to show HMRC a genuine R&D claim is coming, not a full technical case.

It takes an afternoon, not a week. The difficulty is never completing it — it is remembering it exists before the six-month window closes.

Loss-making and R&D-intensive companies have more to lose

If your company is loss-making and at least 30% of total expenditure is on qualifying R&D, you may fall under Enhanced R&D Intensive Support (ERIS) instead of the merged scheme — an 86% additional deduction on top of the normal 100%, producing 186% enhanced expenditure, with a payable credit of up to 14.5% of the surrenderable loss. That is frequently the single largest source of near-term cash for an early, pre-revenue startup. The notification requirement applies to ERIS claims exactly as it does to merged-scheme claims: miss the six-month window and the relief that was meant to fund the next few months of runway is not available at all, whatever the intensity percentage turns out to be.

What to do this week

  1. Find your last accounting period end date, and count forward six months. That is your notification deadline for this period, if one is needed.
  2. Check whether you claimed R&D relief for either of the last two accounting periods. If yes to either, you are likely exempt from notifying this time — but check the exact three-year rule rather than assuming.
  3. If you have never claimed, or your last claim was more than three years before this period's notification deadline, put the notification form in the diary now, not at tax-return time.
  4. If your company is loss-making and R&D-heavy, check the 30% intensity threshold now — it decides whether ERIS or the standard merged scheme applies, and both need the same notification.
  5. If your current accounting period's notification deadline has already passed and you have not filed, that period's R&D relief is very likely gone. Focus instead on getting the next period notified on time.

None of this changes whether your work qualifies as R&D — that test is exactly as it was before 2023. It changes whether you ever get to make the argument at all. Our post on R&D tax relief for startups covers what qualifies and how the merged scheme is calculated; this one is about not losing the right to claim before you get that far. See also our guide to corporation tax for startups for how the credit interacts with your wider corporation tax position.

Where we help

We track your accounting period end, flag the notification deadline the moment it opens rather than the moment it is nearly too late, and file both the notification and the Additional Information Form as a matter of course on every claim we prepare — so a genuinely good R&D claim is never lost to a form nobody remembered. Banded fixed fees from £49 + VAT a month. Get started.