Found a contractor's statements months after your tax return went in, or realised a whole contractor was left off? The question is how far back you can still put it right. GOV.UK gives sole traders and partners two time limits, and one of them ends on 31 January 2027 for the 2024–25 tax year. This guide sets out both in GOV.UK's own words. Limited companies work differently (see near the end).
The short answer
- Within 12 months of the deadline, you can change the return. GOV.UK says: "You can correct a tax return within 12 months of the Self Assessment deadline, online or by sending another paper return." Its own example: "For the 2024 to 2025 tax year, you'll usually need to change your return by 31 January 2027."
- After that, you write to HMRC. GOV.UK says that if it's more than 12 months after the deadline, you write to HMRC to "claim overpayment relief, if you think you've overpaid tax". And: "You can claim overpayment relief up to 4 years after the end of the tax year it relates to." Overpayment relief isn't a later deadline for the same change. The law lets HMRC refuse a claim if you could have changed the return in time and knew, or ought reasonably to have known, that you could (Taxes Management Act 1970, Schedule 1AB, paragraph 2). If you're still within the 12 months, change the return.
Changing a return doesn't always mean money back. GOV.UK says: "Your bill will be updated based on what you report. You may have to pay more tax or be able to claim a refund."
Why CIS deductions belong on the return
GOV.UK's pay tax and claim back deductions page calls CIS deductions "advance payments towards your tax". A sole trader or partner records them on the Self Assessment return for the year:
- "your total pay before deductions were taken, as 'income'"
- "the total deductions contractors have taken, as 'CIS deductions'"
HMRC then works out the tax and takes off the deductions. So if a contractor's statements were missed, two figures were usually missed: some pay, and the deductions taken from it. Correcting the return means looking at both.
The time limits, year by year
The 12-month limit and the 2024–25 date below are GOV.UK's. The 4-year column applies GOV.UK's rule ("up to 4 years after the end of the tax year") to each tax year, which ends on 5 April. Check the date for your year with HMRC before you rely on it.
| Tax year | Online return deadline | Change the return by (usually) | Overpayment relief, 4 years after the year ends (only once the return can't be changed) |
|---|---|---|---|
| 2025–26 | 31 January 2027 | 31 January 2028 | 5 April 2030 |
| 2024–25 | 31 January 2026 | 31 January 2027 | 5 April 2029 |
| 2023–24 | 31 January 2025 | 31 January 2026 (passed) | 5 April 2028 |
| 2022–23 | 31 January 2024 | 31 January 2025 (passed) | 5 April 2027 |
GOV.UK says "usually". The 12 months run from the Self Assessment deadline, so if you registered late and HMRC gave you a later deadline, your dates may differ.
Changing a return online
GOV.UK's change your return page says you must wait 3 days (72 hours) after filing before you update an online return. Then:
- Sign in to your account.
- From "Your tax account", choose "Self Assessment account".
- Choose "More Self Assessment details".
- Choose "At a glance" from the left-hand menu.
- Choose "Tax return options".
- Choose the tax year for the return you want to amend.
- Go into the return, make the corrections and file it again.
GOV.UK says you'll see your amended bill "straight away". Within 3 days, your statement also shows the difference from the old bill and any interest. If you're owed tax, you "may be able to claim a tax refund".
For a paper return, GOV.UK says to send the corrected pages with "amendment" written on each one, plus your name and UTR. If you used commercial software, contact the software provider.
Claiming overpayment relief by letter
Once the 12 months have passed, the change has to be made by letter. GOV.UK says your letter must include:
- that you're making a claim for overpayment relief
- the tax year you think you paid too much tax for
- why you think you've paid too much tax
- how much you think you've overpaid
- whether or not you've already made an appeal for the same payment
If the pay from those statements wasn't on your return either, GOV.UK says the letter also needs to "report income you did not include in your tax return", with the tax year you're correcting, why you think you've paid too little tax, how much you think you've underpaid, and your signature.
GOV.UK warns: "If you do not include all of this information your claim will be rejected." The letter also needs a signed declaration that the details are "correct and complete to the best of your information and belief". If you're claiming for yourself, you sign it. GOV.UK says no one else, including a tax agent, can sign on your behalf. You can say in the letter how you want to be repaid.
Worked example (made-up figures)
Lee is a sole trader. Every figure here is invented.
2024–25. Lee sent his return online in January 2026, but left out one contractor by mistake. Its statements show £3,000 of pay before deductions and £600 of CIS deductions (20%, no materials). To add them, Lee changes the 2024–25 return online. That must usually happen by 31 January 2027. GOV.UK's instructions cover both figures: the £3,000 goes in as income and the £600 as CIS deductions. HMRC's recalculation then shows whether Lee owes more or gets money back.
2022–23. Lee also finds statements from 2022–23 showing £400 of deductions he never included. The pay itself was on his return, so only the deductions are missing. The 12 months for changing that return ended on 31 January 2025. So this one goes in a letter claiming overpayment relief, with every item GOV.UK lists. Applying the 4-year rule, 2022–23 ended on 5 April 2023, so the claim needs to reach HMRC by 5 April 2027.
In both cases the starting point is the same: complete totals for the year, from every statement.
If you never sent a return for that year
This guide is about returns that were sent and need correcting. If you never sent a return for a year, GOV.UK's deadlines page says "The deadline for earlier tax returns has passed and you may have to pay a penalty. Send your tax return or payment as soon as possible". Contact HMRC about how your CIS deductions for that year are handled. GOV.UK's online filing page also says that SA100 forms for 2020–21 or earlier come from the National Archives.
Finding the old statements
GOV.UK's get paid page says: "If you lose any statements, ask your contractor for a replacement." If you can't get them, for example because the contractor has gone out of business, you can write to HMRC (PT Operations, HM Revenue and Customs, BX9 1BX). Include your name, address and UTR, the contractor's name, address and tax reference if you know it, the payment dates or tax months, and why you don't have the statements.
Limited companies are different
A limited company doesn't claim CIS deductions through Self Assessment. GOV.UK's limited company refund guidance says "You can claim for a tax year once it has ended". It adds: "For the past 6 tax years, you can claim online or by post. For earlier tax years, you must claim by post."
FAQ
Can I change my 2024–25 return after 31 January 2027? GOV.UK says that if you miss the deadline, "you'll need to write to HMRC", for example to claim overpayment relief.
I amended my return. When will I hear? HMRC's check when you can expect a reply tool covers Self Assessment and CIS, and GOV.UK says it's updated weekly. Your refund may show as "pending" first, which GOV.UK says "means that the refund has been created but still needs to be approved and paid".
Will an amended refund be paid if I owe a payment on account? GOV.UK's refund page says you may not get a refund "if you have tax due in the next 45 days (for example for a payment on account)". Instead, the money is taken off what you owe.
Do I need to keep the statements after claiming? GOV.UK says: "You need to keep evidence of the tax you think has been overpaid for the relevant period as HMRC may ask for this later."
Rebuilding a year's totals
If you have a folder of old statements (PDFs or photos), cissplit reads them, checks the deduction rate on each one and flags gaps between a contractor's months. Then it totals each tax year, with every statement listed by contractor. Your first 3 statements are free, with no sign-up. To check a single deduction by hand, the free CIS deduction check works through GOV.UK's rule. cissplit doesn't change or send your return, and you still check the figures against your statements.
This isn't tax advice. Check GOV.UK's guide to changing your return or an accountant for your situation, and check the figures before you submit anything.