Correct your Self Assessment return

If you spot a mistake on a Self Assessment return you have already submitted, you should correct it as soon as possible. Errors can include wrong income figures, missed expenses, incorrect tax relief claims, or simple data entry mistakes.

How you correct the error depends on when you discovered it. There is a 12-month amendment window during which you can change the return yourself online or on paper. After that window closes, you must write to HMRC to claim overpayment relief or report underpaid tax.

When this applies

This guide applies if you have already filed a Self Assessment return and need to change it because:

• You entered income or expenses incorrectly
• You forgot to include a source of income (rental, dividends, freelance work)
• You claimed the wrong tax relief or allowance
• You made a calculation error
• You received updated figures after filing (for example, a corrected P60)
• You need to add or remove supplementary pages (SA103, SA105, SA108)

The amendment window

HMRC gives you 12 months from the filing deadline to amend your return. The filing deadline for online returns is 31 January following the end of the tax year. This means you have until the next 31 January after the original deadline to make changes.

You can amend your return as many times as you need within this window. Each amendment replaces the previous version.

How to amend your return online

If you are within the 12-month amendment window, amending online is the quickest method. HMRC processes online amendments within about 4 weeks.

How to amend on paper

If you originally filed a paper return, or prefer not to use the online service, you can send a corrected paper return to HMRC. Mark it clearly as an amendment.

• Complete only the pages that have changed, plus the main SA100 front page
• Write "AMENDMENT" clearly at the top of each page
• Include a brief note explaining what you have changed and why
• Post to: HM Revenue and Customs, Self Assessment, BX9 1AS

Paper amendments take approximately 12 weeks to process. HMRC will write to you confirming the changes and any revised tax calculation.

After the amendment window closes

Once the 12-month window has passed, you cannot amend your return online or on paper. Instead, you must write to HMRC directly.

If you have overpaid tax

You can claim overpayment relief within 4 years of the end of the tax year. Write to HMRC explaining:

• Which tax year is affected
• What the error was
• How much tax you believe you have overpaid
• How you calculated the correct figure

Send your letter to: HM Revenue and Customs, Self Assessment, BX9 1AS. Include supporting documents such as corrected P60s, bank statements, or revised accounts.

If you have underpaid tax

If the error means you owe HMRC more tax, write to them explaining the mistake. Pay the additional tax as soon as possible. HMRC will charge interest from the original due date, but voluntarily disclosing an underpayment is treated far more favourably than waiting for HMRC to discover it.

Effect on payments on account

If your amendment changes your tax liability, it may also change your payments on account for the following year. Payments on account are advance payments towards next year's tax bill, calculated at 50% of the previous year's liability.

• Amendment increases your tax: Your next payments on account will increase. You may also owe a balancing payment for the current year.
• Amendment reduces your tax: Your payments on account will decrease. You may be able to apply to reduce future payments on account if you expect lower income.

Check your HMRC online account after the amendment is processed to see updated payment dates and amounts.

HMRC discovery assessments

Even after the amendment window and the standard enquiry window close, HMRC can issue a discovery assessment if they believe insufficient tax was assessed. The time limits depend on the nature of the error:

• 12 months from filing: HMRC can open a standard enquiry into any return
• 4 years: HMRC can reassess if they discover an innocent or careless error
• 6 years: For careless errors where HMRC has specific grounds
• 20 years: For deliberate errors or fraud

A discovery assessment requires HMRC to show that they could not reasonably have been expected to be aware of the insufficiency from the information you provided. If you disclosed everything accurately on your original return, HMRC's ability to make a discovery assessment is limited.

Interest on corrections

Amendments that change your tax liability trigger interest adjustments:

• Underpayment: HMRC charges interest from the original payment due date (31 January) until you pay. The current rate is base rate plus 4%.
• Overpayment: HMRC pays repayment interest from the date you made the payment until the refund is issued. The repayment interest rate is lower than the late payment rate.

Interest is not a penalty -- it compensates for the time value of money. It applies regardless of whether the error was deliberate or accidental.

Common situations

Forgot to include freelance income

If you earned freelance or side income and did not include it on your return, amend as soon as possible. You may need to add SA103 (self-employment) supplementary pages. This is a common error that HMRC can detect through third-party data matching.

Claimed wrong mileage or expenses

If you over-claimed or under-claimed business expenses, amend the relevant section. Keep your original receipts and mileage logs as evidence.

Received a corrected P60

If your employer issues a corrected P60 after you have filed, amend your return to reflect the updated figures. Your employer should also update their RTI submissions with HMRC.

Marriage Allowance or Blind Person's Allowance

If you forgot to claim an allowance you are entitled to, or claimed one you should not have, amend the relevant section. For Marriage Allowance, both you and your partner's returns may need amending.