R&D: The 2-Year Claim Window: When You Can Still Claim Past Periods in the UK
- MAZ
- Apr 14
- 16 min read
The 2-Year Claim Window: Why It’s More Important Than Ever for UK Businesses in 2026
Picture this: You’re sitting with your accountant in early 2026, reviewing last year’s figures, and suddenly realise your engineering team spent months cracking a genuine technological uncertainty back in 2024. The work qualified for R&D relief – but has the window closed?
None of us enjoys that sinking feeling. In my 18 years advising UK companies on R&D claims, I’ve seen far too many businesses leave serious money on the table simply because they didn’t understand exactly when the clock stops ticking. The good news? For many of you reading this in 2026, the two-year window is still wide open for recent past periods – but only if you act correctly and quickly.
How the two-year rule actually works in practice
The rule itself is straightforward once you strip away the jargon. R&D tax relief (whether under the merged R&D Expenditure Credit scheme or the old rules that still apply to earlier periods) is claimed through your corporation tax return or an amendment to it. You have two full years from the end of the accounting period (or “period of account”) in which the qualifying expenditure was incurred.
For a standard 12-month accounting period ending on, say, 31 March 2024, that means you can still submit or amend your claim right up to midnight on 31 March 2026. Miss that date and the relief for that specific period is gone forever in almost every case.
The legislation sits in Schedule 18 to the Finance Act 1998 (paragraph 83E if you want the exact reference). HMRC has always been crystal clear: the clock starts the day after the accounting period ends and runs for exactly 24 months for periods of account that are 18 months or shorter. Longer periods have their own formula, but most companies never hit that.
Why 2026 feels different – and more urgent
With the merged R&D scheme now fully bedded in for accounting periods beginning on or after 1 April 2024, many businesses are finally getting their heads around the new 20% above-the-line credit and the Enhanced R&D Intensive Support (ERIS) rules. But here’s the bit that catches people out: the two-year window applies equally to pre- and post-merger periods. So right now in 2026 you can still claim for qualifying spend incurred in 2024 (and sometimes late 2023) under whichever scheme applied at the time.
I had a manufacturing client last month who thought the merged scheme “reset everything”. It doesn’t. The time limits stayed exactly the same – which is why reviewing older periods is one of the highest-value exercises you can do this year.
Calculating your exact claim window – let’s make it personal
The deadline is always tied to the end of your own accounting period, not the tax year. Here’s how it looks for the most common year-ends businesses are dealing with in 2026:
Accounting period end date | Latest date to submit or amend claim | Still open in March 2026? |
31 March 2024 | 31 March 2026 | Yes – closing fast |
30 June 2024 | 30 June 2026 | Yes |
30 September 2024 | 30 September 2026 | Yes |
31 December 2024 | 31 December 2026 | Yes |
31 March 2023 | 31 March 2025 | No – closed |
If your year-end falls between these, just add exactly 24 months to the last day of the period. For periods of account longer than 18 months the deadline becomes 42 months from the first day – but again, that’s rare.
The hidden six-month gatekeeper that can invalidate even timely claims
Here’s where most articles stop – and where I see businesses come unstuck. Since accounting periods beginning on or after 1 April 2023, HMRC introduced a mandatory Claim Notification Form (CNF). If this is your first-ever R&D claim, or you haven’t made a valid claim in any of the previous three accounting periods, you must tell HMRC you intend to claim within six months of the end of the period of account.
Miss that six-month window and your claim becomes invalid – even if you’re still inside the two-year amendment period. I’ve watched a perfectly good £85,000 claim for a 2024 period get rejected purely because the notification was filed a week late. The client was devastated.
The notification period starts on the first day of your period of account and ends exactly six months after the last day. For a 1 January to 31 December 2024 period, that meant notification had to be in by 30 June 2025. Simple maths, but easy to overlook when you’re busy running the business.
What this means for claims on past periods right now
If you’re reading this in March 2026 and your accounting period ended on or after 1 April 2024, check whether you submitted the CNF at the time. If not, and you haven’t claimed R&D in the three years before that period, you may already be locked out – even though the two-year window hasn’t closed yet.
The Additional Information Form (AIF) is another non-negotiable that must be submitted before or on the same day as your corporation tax return. Submit the return first and the claim gets stripped out. I’ve seen companies lose relief worth tens of thousands simply because the forms went in the wrong order.
Be careful here – especially if you’re a first-time claimant or haven’t claimed recently. The rules were brought in to reduce error and fraud, but they’ve caught plenty of genuine businesses who simply didn’t know the process had changed.
Now you know exactly where your window sits
The next step is working out whether qualifying expenditure from those still-open periods is sitting hidden in your accounts. In the second part of this guide I’ll walk you through a practical checklist I give every client who comes to me wondering “have we missed something?” – plus the most common pitfalls that trip people up when they try to claim for past periods under the merged scheme rules.
Because in my experience, the businesses that act now on 2024 and early 2025 periods are the ones that turn compliance headaches into welcome cashflow boosts. None of us can change past deadlines – but we can still make the most of the ones that remain open.
Reviewing Past Periods: A Practical Checklist to Uncover Missed R&D Relief
Right, you’ve got your head around the deadlines – now the real work starts. In my experience, the biggest wins come from companies who sit down and methodically trawl through their last couple of years’ accounts looking for qualifying spend they never flagged as R&D. Especially in 2026, with so many periods still open under the merged scheme, this exercise can deliver genuine cash back into the business.
I always tell clients: treat this like a health check. You wouldn’t ignore symptoms in your company finances, so don’t ignore potential R&D that’s hiding in plain sight.
Step 1: Map your open accounting periods first
Before you dive into the detail, list every period of account that’s still within the two-year window. Use your year-end dates – not the calendar year or tax year.
For example, if your accounts run April to March:
● Period ended 31 March 2024 → claim/amend by 31 March 2026 (closing very soon now in early 2026)
● Period ended 31 March 2025 → claim/amend by 31 March 2027
Cross-reference against whether you submitted the Claim Notification Form (CNF) within six months of each period ending. If you didn’t – and this wasn’t a follow-on claim within three previous periods – that period is already blocked, even if the two-year window hasn’t shut.
I’ve had clients discover mid-review that their 2024 period was lost purely because the CNF was forgotten. Painful, but better found now than after you’ve spent weeks gathering evidence.
Step 2: The “red flags” scan – where R&D usually hides
Run through your nominal ledger, project codes, timesheets and supplier invoices with these common triggers in mind:
● Software development that went wrong or needed major iteration
● Prototyping or testing new materials/processes where the outcome wasn’t certain
● Engineering tweaks to improve performance, yield or reliability beyond routine application
● App or system integration that required solving a genuine tech uncertainty
Be careful here – HMRC is strict on the “uncertainty” test. It’s not enough that the work was difficult; there must have been a scientific or technological hurdle that competent professionals couldn’t easily resolve. I’ve seen claims rejected because the narrative was too vague – “improved efficiency” without explaining the barrier.
A quick checklist I hand clients (print this out or copy it)
● Did we incur staff costs on projects involving experimentation or systematic trial/error? (Include time spent by directors if they’re hands-on)
● Any subcontracted work to solve a specific technical problem? (Note: subcontractor rules changed under the merged scheme – only 65% of qualifying subcontractor costs count unless they’re connected)
● Consumables used up in testing/prototyping? (Heat, light, power apportioned to the R&D phase can qualify)
● Software licences or cloud computing purely for the R&D activity?
● Clinical trials data analysis or similar where the protocol hit genuine unknowns?
Tick yes to three or more and you almost certainly have something worth digging into deeper.
Step 3: Quantify the spend – avoid common costing pitfalls
This is where many go wrong. People either over-claim (and get hammered in enquiry) or under-claim out of caution. My rule of thumb from 18 years: be robust but realistic.
Use this simple framework for apportionment:
Direct staff – full time on qualifying project = 100%
Mixed roles – use timesheets or reasonable estimate (I prefer contemporaneous records; retrospective ones are scrutinised more)
Overheads – apportion based on floor space, headcount or machine hours only for the R&D portion
Externally provided workers – now limited under merged rules
For periods before 1 April 2024 (still claimable now if open), check which scheme applied: old SME (enhanced deduction) or old RDEC. The merged scheme applies only from periods beginning on or after that date, but transitional rules can be fiddly for straddling periods.
Real client example – the late discovery that paid off
Last autumn I worked with a precision engineering firm whose year-end is 30 September. Their 30 September 2024 period closed for amendment on 30 September 2026 – plenty of time. But they’d never claimed before.
They’d spent heavily on CNC machining trials for a new alloy in 2023–24, hitting genuine uncertainties around heat treatment consistency. No CNF submitted by 31 March 2025 (six months after period end). Result: claim invalid despite being inside the two-year window.
We couldn’t save that period, but we salvaged their 30 September 2025 period (CNF still open) and identified carry-over qualifying work. Net result: £62,000 credit under the merged scheme instead of zero. The lesson? Always file the CNF even if you’re “not sure yet” – it buys you the option.
Pitfalls that trip people up when claiming past periods
● Assuming the merged scheme rates apply backwards – they don’t. Pre-1 April 2024 periods use old rules.
● Forgetting the Additional Information Form (AIF) must precede or accompany the CT return/amendment. File the return first and the R&D claim vanishes.
● Over-relying on “routine” engineering as qualifying – HMRC loves challenging this. Use the BEIS guidelines definition religiously.
● Missing subcontractor apportionment changes – under merged scheme, connected party subcontractors are treated differently.
● Poor narrative – “we innovated” won’t cut it. Describe the uncertainty, attempts to resolve, outcome.
What if HMRC opens an enquiry?
They can – and do – within 12 months of the claim date usually. Have your project records ready: meeting notes, test logs, emails about problems solved. I’ve seen claims reduced by 40–60% where documentation was thin.
In one First-tier Tribunal case (involving late or invalid claims broadly), the tribunal upheld HMRC’s strict application of time limits where no exceptional circumstances existed. Late claims rarely succeed unless you can show genuine external factors beyond your control – don’t bank on leniency.
Wrapping up the review process
Once you’ve identified spend, drafted a solid technical narrative, and run the numbers, decide: amend the return yourself via your software, or get specialist help if the amount justifies it. For larger claims, early discussion with HMRC via their advance assurance pilot (rolling out in 2026 for SMEs) can give peace of mind.
In the final part, I’ll cover the merged scheme specifics for 2025/26 periods, ERIS eligibility traps, and a summary of the key actions you should take right now in March 2026.
Because the window closes faster than most realise – and the money left unclaimed rarely comes back.
The Merged Scheme in 2026: Turning Open Windows into Real Cash
Now that you’ve mapped your open periods and run the checklist from the last section, it’s time to apply the current rules properly. For any accounting period that began on or after 1 April 2024 and is still within the two-year amendment window, the merged R&D Expenditure Credit (RDEC) rules apply – a flat 20% above-the-line credit for every pound of qualifying spend, regardless of company size.
The beauty (and the trap) is that the relief hits your profit-and-loss account before tax, so profitable companies get an immediate corporation tax saving of around 15–16.2% net after tax. Loss-makers can surrender it for a cash credit. But only if every box is ticked – including the Additional Information Form filed in the right order.
I’ve seen clients in early 2026 breathe a huge sigh of relief when we confirmed their 30 September 2024 period (still open until September 2026) qualified under the merged scheme and delivered a credit they’d almost written off. The key? We double-checked the qualifying expenditure against the stricter merged-scheme definitions before amending the return.
ERIS: The enhanced route that many loss-making SMEs are still missing
If you’re a loss-making SME with genuinely intensive R&D, don’t default to the merged scheme. Enhanced R&D Intensive Support (ERIS) can deliver far more – up to an effective 27%+ benefit in the right circumstances.
The threshold dropped to 30% of total expenditure from accounting periods beginning on or after 1 April 2024 (it was 40% before). That change alone has brought dozens more companies into the higher relief bracket this year.
Here’s how the two options compare right now in 2026:
Scenario | Scheme | Effective benefit (profitable) | Effective benefit (loss-making) | Key condition |
Standard claim | Merged RDEC | ~15–16.2% | Cash credit option | All companies |
R&D-intensive loss-maker | ERIS | Not available | Up to ~27% (enhanced deduction + repayable credit) | ≥30% R&D intensity |
The intensity ratio is calculated as qualifying R&D spend divided by total expenditure (including group companies where relevant). Get this wrong and you lose ERIS entirely – even if you’re otherwise eligible.
Subcontractor and externally provided worker rules – the 2026 changes still catching people out
Under the merged scheme the old SME subcontracting generosity has gone. You can now only claim 65% of payments to unconnected subcontractors (or the lower of cost or 65% for externally provided workers). Connected-party rules are tighter too.
Be careful here if your 2024 or 2025 projects relied heavily on specialist labs or freelance developers. I had one software client last month who had to reduce their claim by £28,000 after we reapplied the merged-scheme apportionment – but we still salvaged £41,000 net because we caught it before filing.
The flip side? If you’re the subcontractor doing the R&D, you can now claim under the merged scheme yourself – a route many smaller firms are using successfully in 2026.
Advanced pitfalls that even experienced directors fall into
None of us enjoys tax surprises, but these three keep cropping up with past-period claims:
● Treating routine product development as qualifying – the “competent professional” test is applied more rigorously under merged rules.
● Forgetting to exclude subsidised expenditure correctly (government grants now interact differently).
● Poorly documented cloud-computing or data costs – HMRC wants clear apportionment to the R&D phase only.
One First-tier Tribunal decision in recent years upheld HMRC’s strict view on claim validity where notification and information requirements were missed, reinforcing that exceptional circumstances are needed to extend deadlines. Don’t assume leniency.
Your March 2026 action plan – the exact steps I give paying clients
Confirm every open period’s end date and two-year deadline today.
Check ERIS eligibility using last two years’ accounts – the calculation is fiddly but worth it.
Draft or update your technical narrative using the official BEIS guidelines wording.
Gather AIF data (project descriptions, costs breakdown, uncertainties) before touching the CT return.
File any outstanding Claim Notification Forms immediately if the six-month window is still open.
Run the numbers through approved software or ask your accountant for a shadow calculation.
Submit amendments in the correct order – AIF first, then CT600.
Keep full records for at least six years – enquiries are rising.
Consider HMRC’s advance assurance service if your claim is over £50,000.
Diarise the next period’s CNF deadline the moment your accounts close.
Follow these and you’ll sleep easier knowing you’ve maximised every still-open period without triggering unnecessary scrutiny.
Summary of Key Insights
The two-year amendment window is fixed to your accounting period end date and remains unchanged in 2026.
Miss the six-month Claim Notification Form deadline on a first or non-recent claim and the entire period is lost, even inside the two-year window.
The merged scheme’s 20% above-the-line credit applies to all open periods beginning on or after 1 April 2024 – but only the qualifying costs under the new definitions.
Loss-making SMEs meeting the 30% R&D intensity test should elect for ERIS to unlock significantly higher effective relief.
Subcontractor costs are capped at 65% for unconnected parties under merged rules – a frequent source of over-claiming.
The Additional Information Form must be submitted before or with the corporation tax return or amendment, otherwise the claim is rejected.
Retrospective timesheets and vague project narratives remain the top two reasons claims are reduced or refused.
ERIS intensity is calculated on total expenditure including connected companies – get the ratio wrong and you default to the lower merged rate.
HMRC enquiries on past-period claims are increasing in 2026; contemporaneous records are your best defence.
Acting now on any 2024 or early 2025 periods still open can turn compliance into genuine positive cashflow – the windows are closing faster than most realise.
There you have it – everything you need to review, recalculate and claim with confidence in March 2026. If any of this feels close to home, drop your year-end dates and rough spend figures into a quick note to your accountant this week. None of us enjoys leaving money behind, and with the rules now settled, there’s never been a better time to tidy up those past periods properly.
FAQs
Q1: Can a company still make a valid R&D claim if it missed the six-month claim notification but is inside the two-year window?
Well, it’s one of those frustrating technicalities that catches even careful directors out. In practice, if this is your first claim or you haven’t claimed in the previous three accounting periods, the notification is a hard gatekeeper. Miss it and the claim is invalid no matter how much time remains in the two-year window. I once worked with a precision engineering firm in Birmingham that had solid qualifying spend but filed their notification seven weeks late – the entire claim was wiped out despite being well inside the amendment period. The only slim hope is proving genuine exceptional circumstances to HMRC, but that’s rare and never guaranteed.
Q2: Is there any realistic way to extend the two-year claim window for genuinely exceptional reasons?
In my experience over 15 years, extensions are exceptionally rare and almost never granted just because you “didn’t realise” or were too busy. HMRC and the tribunals take a strict line – think late evidence or external disasters like a fire destroying records. One recent tribunal case involved a company arguing administrative chaos after a merger; the court refused the late claim outright. If you’re in this boat, act immediately with professional evidence rather than hoping for leniency.
Q3: How does switching accounting periods mid-year affect the deadline for past R&D expenditure?
This one trips up growing businesses that change year-ends. The two-year clock is tied strictly to each individual period of account, not the tax year. So if you shortened or lengthened a period, you must calculate the exact end date for that specific slice of expenditure. I had a client in Manchester who changed from 31 December to 31 March and nearly missed the window for six months of qualifying software development because they applied the wrong end date. Always map every period separately – it’s the only safe way.
Q4: What happens to past R&D claims if the company is acquired or goes through a group restructure?
The right to claim stays with the company that incurred the expenditure, even after an acquisition. The new owners can still amend the old returns within the original two-year window. However, you must be careful with group relief interactions and connected-party rules under the merged scheme. I’ve seen new parent companies successfully claim for pre-acquisition work, but only when the records were properly handed over – messy due diligence often costs valuable relief.
Q5: Can a dormant company or one with zero turnover still claim R&D relief for past periods?
Absolutely yes, provided the qualifying activity happened and the company was trading or preparing to trade at the time. Dormancy doesn’t kill the claim window. A software startup I advised spent its first 18 months in R&D with no sales – they successfully claimed for two past periods once revenue finally started. The key is proving the expenditure was incurred for qualifying purposes, not routine admin.
Q6: How does the two-year rule work when an accounting period is longer than 12 months?
For periods over 12 months the deadline is still two years from the end of that long period, not prorated. It’s a common misconception that leads to early panic. One manufacturing client had an 18-month period ending in June 2024 – their window closes in June 2026, full stop. Just remember the claim notification six-month rule still applies from the actual end date.
Q7: Is it possible to correct or increase an already-filed R&D claim once the two-year window has passed?
No, once that window closes the position is final – no amendments, no top-ups, no second chances. That’s why I always advise clients to over-prepare rather than rush. I’ve had directors come to me 15 months after the deadline wanting to add overlooked subcontractor costs – heartbreakingly, there’s nothing we can do. File conservatively within time, then amend upwards before the clock stops.
Q8: What should a company do immediately if HMRC rejects a timely R&D claim for missing additional information?
Don’t panic, but act fast. You usually have 30 days to appeal or request a review. Gather the missing AIF data, technical narrative and cost breakdowns immediately and submit a formal appeal. In one case a biotech client in Cambridge had their claim knocked back for incomplete project descriptions – we resubmitted with clearer uncertainty evidence and recovered the full credit plus interest. Speed and quality of response make all the difference.
Q9: Does the two-year window apply the same way if the company has since ceased trading or entered liquidation?
The window doesn’t extend, but you can still claim up to the normal deadline even in liquidation. The liquidator must file the amendment. I’ve assisted two companies in voluntary liquidation where we squeezed in last-minute claims for earlier periods – the cash went to creditors rather than shareholders, but it still mattered. Leave it any later and the relief disappears forever.
Q10: Can a company claim R&D relief for work carried out in a previous accounting period that straddled the 1 April 2024 merged scheme start date?
Yes, but you have to apportion carefully between old and new rules. Expenditure before 1 April 2024 follows the old SME or RDEC rules; after follows the merged scheme. It’s fiddly and many businesses get the split wrong. A client with a March year-end had to rework their claim entirely when we discovered the apportionment error – we still salvaged a decent credit but lost time.
About the Author
Maz Zaheer, AFA, MAAT, MBA, is the CEO and Chief Accountant of MTA and Total Tax Accountants, two premier UK tax advisory firms. With over 15 years of expertise in UK taxation, Maz provides authoritative guidance to individuals, SMEs, and corporations on complex tax issues. As a Tax Accountant and an accomplished tax writer, he is renowned for breaking down intricate tax concepts into clear, accessible content. His insights equip UK taxpayers with the knowledge and confidence to manage their financial obligations effectively.
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