Can I Amend My Self Assessment Tax Return After Filing It? A Complete Guide for 2026

Yes — you can amend your Self Assessment tax return after you have submitted it, whether you spotted a mistake, forgot to include income, claimed the wrong expenses, or simply entered incorrect figures. HMRC provides a straightforward process to correct most errors, helping you avoid penalties and interest charges.

Updated March 2026: HMRC allows you to make changes to your Self Assessment return within 12 months of the original filing deadline (usually 31 January). For the 2025/26 tax year (deadline 31 January 2027), you can amend online until 31 January 2028. After this window, you can still request changes by writing to HMRC, but the process is more formal and you may face interest or penalties if the amendment increases your tax bill.

At Filing Accounts, we help many self-employed individuals, landlords, and company directors correct their tax returns quickly and accurately, minimising any extra costs. Contact us today for expert assistance with amending your Self Assessment return.

Why Would You Need to Amend Your Self Assessment Tax Return?

Everyone makes mistakes — even careful people. Common reasons for amending a tax return include:

Forgetting to declare some income (e.g., freelance work, rental income, or bank interest)
Missing allowable expenses or claiming too much
Entering incorrect figures for dividends, pensions, or capital gains
Simple typing errors in personal details, addresses, or calculations
Discovering new information after filing (e.g., a late-issued invoice or P60)

Making an amendment promptly is always better. It shows HMRC you are acting honestly and can reduce or avoid penalties if the change increases the tax you owe.

Example: You filed your 2025/26 return on 20 January 2027 but later realised you forgot £3,000 of freelance income. Amending within the 12-month window lets you correct this easily before HMRC’s data-matching systems flag it.

How Long Do You Have to Amend Your Self Assessment Return?

The standard window is 12 months from the Self Assessment filing deadline for that tax year.

For the 2025/26 tax year (filing deadline: 31 January 2027) → You can amend until 31 January 2028.
Online filers must wait 72 hours (3 days) after submission before the amendment option becomes available.

If you miss the 12-month window, you can still request a change by writing to HMRC, but this is treated as a formal claim or disclosure. HMRC may accept it up to 4 years later for overpayments (or longer in cases of genuine error), but interest will usually apply on any additional tax due.

Important: Acting quickly is always best. The sooner you correct underpaid tax, the less interest accrues.

Official GOV.UK guidance: Correcting your Self Assessment tax return

How to Amend Your Self Assessment Tax Return (Step-by-Step)

Within the 12-Month Window (Easiest Method)

Wait 72 hours after you originally submitted the return.
Log into your HMRC Personal Tax Account via Government Gateway.
Go to Self Assessment → Tax returns and calculations.
Select the relevant tax year and choose Amend return.
Make the necessary changes carefully.
Review the updated tax calculation (HMRC will automatically recalculate your bill or refund).
Submit the amended return.

You will receive immediate confirmation, and any change to your tax liability will be shown straight away.

After the 12-Month Window

You must write a letter to HMRC including:

Your full name and National Insurance number
The tax year(s) affected
A clear description of the error and the correct figures
Whether it results in more tax due or a refund
Your contact details and signature

Send the letter to: Self Assessment, HM Revenue and Customs, BX9 1AS, United Kingdom

HMRC will review your request and reply, which may take several weeks.

Pro tip: Always keep a copy of your letter and any supporting documents.

What Happens When You Amend Your Return?

HMRC recalculates your tax automatically.
If you owe more tax → You must pay the difference plus any interest from the original due date.
If you are due a refund → HMRC will usually repay it to your nominated bank account.
Amending within the 12-month window generally avoids penalties if done voluntarily and honestly.

Common Mistakes and How to Avoid Them

Forgetting the 72-hour waiting period after filing.
Making changes without double-checking the impact on your overall tax bill.
Amending only one year when multiple years are affected.
Delaying the amendment until HMRC contacts you (this can trigger higher penalties).

What Our Clients Say on Trustpilot

“Filing Accounts spotted an error in my Self Assessment and amended it quickly. Saved me from extra interest!” – Anonymous, March 2026 (5 stars)
“Clear guidance on amending my return. They handled everything professionally.” – Mark T., February 2026 (5 stars)
“Stress-free process for correcting my tax return. Highly recommend for self-employed.” – Sarah L., January 2026 (5 stars)

With our consistent 4.2/5 Trustpilot rating, clients trust us to fix tax return issues efficiently.

Frequently Asked Questions

How long after filing can I amend my Self Assessment return? You have 12 months from the original filing deadline (usually 31 January).

Can I amend a paper tax return? Yes — you can submit a new paper return or write to HMRC.

Will amending my return trigger a tax investigation? Voluntary amendments within the time limit rarely do. However, large or repeated changes may attract HMRC attention.

What if the amendment means I owe more tax? You will need to pay the additional amount plus interest from the original due date.

Can Filing Accounts help me amend my Self Assessment? Yes — we review your return, make accurate amendments, and handle communication with HMRC.

Official GOV.UK resources:

Need Help Amending Your Self Assessment Tax Return?

Discovering a mistake on your tax return can be worrying, but it is usually easy to fix. Filing Accounts provides clear advice, accurate amendments, and full peace of mind so you stay compliant and avoid unnecessary penalties or interest.

Contact us today or book a free consultation — we’ll review your situation and correct your Self Assessment return efficiently in 2026.