The 12-month amendment window
You can amend a filed UK Self Assessment return within 12 months of the original filing deadline. Beyond that window, overpayment relief claims are possible for up to 4 years but with stricter conditions. This guide covers exactly how to amend, when to use overpayment relief instead, and the consequences of getting an amendment wrong.
The 12-month amendment window
You can amend a filed Self Assessment return within 12 months of the original 31 January online filing deadline. So the 2024/25 return (deadline 31 January 2026) can be amended until 31 January 2027.
Any reason for amendment: found a missed expense, forgot pension relief, discovered an error in income calculation. HMRC recalculates the tax and adjusts your balance.
How to amend online
Sign in to your personal tax account with Government Gateway. Navigate to Self Assessment → view submitted returns → select the tax year to amend.
Edit the affected sections. HMRC's system shows before/after calculations. Save the amendment; submit.
Outcome: if amendment reduces your tax bill, HMRC issues a refund (usually within 2-6 weeks). If amendment increases your bill, you'll owe the difference plus interest from the original 31 January deadline.
When to use overpayment relief instead
After the 12-month amendment window, you can claim overpayment relief for up to 4 years after the tax year end (so 2024/25 relief can be claimed until 5 April 2030).
Overpayment relief is stricter — you must have evidence of the overpayment and the reason it wasn't caught in the original return. Typical cases: missed pension relief, forgotten Gift Aid, retrospectively-received rebates.
HMRC-issued form SA1 or written claim. Timelines: 6-12 weeks to process.
Worked example
Marcus filed 2023/24 return on 15 January 2025. In August 2025, reviewing his pension provider statement, he sees a £3,500 personal contribution he'd forgotten to declare. Higher-rate relief available: £3,500 × 20% = £700. Within 12-month window (deadline 31 January 2026). He signs in, amends the return, adds the contribution to the adjustments section. HMRC recalculates, issues £700 refund within 4 weeks. No penalty, no interest — the amendment is standard.
Reference table
| Timing | Route | Difficulty |
|---|---|---|
| Within 12 months of original deadline | Online amendment | Easy — 15 minutes |
| 12 months to 4 years | Overpayment relief claim (SA1 or written) | Medium — 6-12 weeks |
| 4 years+ | Very limited — special circumstances only | Difficult |
| To increase tax owed | Same routes as reduction | Add interest from original due date |
Amendment checklist
- Identify what needs changing and the reason
- Confirm you're still within the 12-month window (or 4-year overpayment relief window)
- Gather evidence for the change (pension statement, receipt, corrected income)
- Sign in with Government Gateway, navigate to the return
- Enter the amended figures
- Review the recalculation before submitting
- Save the amendment receipt
- Diary the expected refund arrival (2-6 weeks) or additional payment
Yes — from the original 31 January due date, not the amendment date. HMRC calculates automatically.
Yes — each amendment supersedes the previous, provided you're still within the 12-month window.
The online system doesn't require an explanation but HMRC may follow up if the amendment is unusual (very large reduction, patterns of amendment). Keep evidence.
HMRC can 'correct' obvious errors (arithmetic, obvious mis-classification). Larger changes generally happen via enquiry, not unilateral amendment.
Amendment is the right route within 12 months. Beyond that, HMRC's Digital Disclosure Service (DDS) handles voluntary disclosures with reduced penalties.
Sources & official references
Related on FreelanceToolkit UK
See also: HMRC Deadlines Calendar 2025/26 — related guide from FreelanceToolkit UK.
See also: UK Self Assessment Checklist 2025/26 — related guide from FreelanceToolkit UK.
See also: UK Tax Rates Reference 2025/26 — related guide from FreelanceToolkit UK.
Related: Client Onboarding Checklist — UK Freelancer — from FreelanceToolkit UK.
Related: New Freelancer Starter Pack — UK 2025/26 — from FreelanceToolkit UK.
Related: Contractor vs Employee UK 2026 — Side-by-Side Comparison — from FreelanceToolkit UK.
Related: UK VAT Rates Reference — from FreelanceToolkit UK.
This guide is general information based on UK rules for the 2025/26 tax year. It is not personal tax or legal advice. For decisions affecting your tax position or legal exposure, consult a qualified accountant or solicitor.