Tax Recovery · 2026
The 4-Year Lookback: How to Claim Back CIS Tax You Never Knew You Were Owed
Most subcontractors assume a missed expense claim is gone forever. It is not — HMRC gives you four years to go back. But every year that passes closes the window on another £720, £1,200, or £4,000 that is yours to recover.
The tax return you filed three years ago may still owe you money. If you failed to claim legitimate expenses — mileage, tools, materials, use of home — you did not necessarily lose that money. You just have not claimed it yet. HMRC's Overpayment Relief window gives you four years from the end of the tax year to go back and correct the record. The window is closing. But it has not closed yet.
The Rule Most Subcontractors Don't Know Exists
When you submit a Self Assessment tax return, HMRC does not treat it as the final word. The law allows you to go back and correct errors or omissions — but only within a defined window. For most CIS-related claims, that window is four years from the end of the relevant tax year.
This is HMRC's Overpayment Relief provision. It exists because HMRC acknowledges that people make mistakes, forget expenses, or simply do not know what they are entitled to claim. It is not a loophole. It is a legal mechanism designed to correct genuine overpayments — and thousands of CIS subcontractors qualify for it every year without ever making a claim.
Every tax year that passes without a lookback review is a year where the window closes permanently on money you may have been owed.
CIS Tax Insights, 2026Which Years Are Still Open Right Now?
As of April 2026, these are the tax years still within the four-year window — and when each one expires:
The window for the 2022/23 tax year closes on 5 April 2027. That is less than 12 months away. Any unclaimed expenses from that year — mileage, materials, tools, use of home — must be claimed before that date or they are permanently lost. This is the year to check first.
What a Backdated Claim Can Actually Recover
This is not theoretical. The numbers below reflect the type of claim that subcontractors make every year when they go back through old records with fresh eyes.
| Business miles driven (never claimed) | 8,000 miles |
| HMRC approved mileage rate | 45p/mile |
| Total unclaimed mileage expense | £3,600 |
| Additional unclaimed expenses (tools, PPE, insurance, use of home) | £2,400 est. |
| Total unclaimed expenses — one year | £6,000 |
| Tax overpaid at basic rate (20%) | £1,200 |
| Recoverable across all 4 open years (estimate) | £1,500 – £4,800 |
What to Look For in Your Old Records
The most commonly missed expenses in backdated CIS claims are not obscure deductions — they are everyday costs that most subcontractors pay but never record.
The Two Ways to Claim: Amend vs Overpayment Relief
How you claim depends on how old the year is. The process is different — but both are legitimate and both can recover real money.
Practical Checklist: How to Run Your Lookback Review
Set aside 30–60 minutes, work through these steps in order, and you will have a clear picture of whether a backdated claim is worth pursuing.
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Identify Which Years Are Still Open Start with 2022/23 — this is the most urgent. List all four open tax years and note the deadline for each. If you use an accountant, ask them to confirm which years have amendment windows still available versus which require Overpayment Relief.
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Pull Your Old Bank Statements Most UK banks provide statements going back 5–7 years online. Download them for each open tax year. Look for payments to trade suppliers, fuel stations, tool retailers, insurance providers, and phone companies. These are your expense categories — even without receipts, bank statements establish that a purchase was made.
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Reconstruct Your Mileage If you did not keep a mileage log, you can often reconstruct business travel using Google Maps history, old job emails or texts, or CIS vouchers that show work locations. HMRC accepts a reasonable estimate supported by corroborating evidence — but "I think I drove about 8,000 miles" without any supporting documentation is not sufficient.
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Compare Against What You Actually Declared Log into your Government Gateway account and download your SA302 for each open year. This shows exactly what expenses you declared. Compare that against the bank statement analysis — the gap between the two is your potential claim. If you cannot access old SA302s, your accountant can retrieve them from HMRC.
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Submit the Claim — or Appoint an Accountant to Do It For years within the 12-month amendment window, submit the amended return yourself via Government Gateway. For older years requiring Overpayment Relief, this is a formal process that benefits from professional handling — particularly because HMRC applies extra scrutiny to backdated claims and incomplete documentation can result in the claim being rejected entirely.
Every week of delay is a week closer to losing money permanently.
The four-year window is not a guideline — it is a hard legal deadline. HMRC does not grant extensions. Once 5 April 2027 passes, the 2022/23 tax year is closed forever. Whatever was overpaid in that year cannot be recovered by any means. That is the only scenario in this entire process where "I'll deal with it later" becomes "I've lost the money." Start the review this week.
QuoteDone Tools
Find Out What HMRC Still Owes You.
Use our free CIS Refund Calculator to check your current year position — then work backwards through the open years before the window closes.
This article is for informational purposes only and does not constitute professional tax advice. Always consult a qualified accountant before submitting a backdated claim or Overpayment Relief request. Deadlines based on 2026 HMRC guidance.