Filing your Self Assessment tax return can feel stressful — and discovering a mistake after submission is even worse. The good news is that in most cases, you can correct a UK tax return after filing without penalties if you act promptly and follow the correct process set by HMRC.
This guide explains how to fix a Mistake on my UK Tax Return after Submission step-by-step, deadlines, penalties, and when you should contact an accountant.
How to Fix a Mistake on My UK Tax Return after Submission
Can You Amend a UK Tax Return After Submission?
Yes. HMRC allows taxpayers to amend a Self Assessment tax return within 12 months of the filing deadline.
Example:
- 2024–25 tax return deadline: 31 January 2026
- Amendment deadline: 31 January 2027
If you’re within this window, you can correct most mistakes online or by post.
Common UK Tax Return Mistakes
Many taxpayers search how to fix tax return errors because of:
- Incorrect income figures
- Missing rental income
- Wrong expense claims
- Forgot to declare side income
- Capital gains omitted
- CIS or employment income errors
- Bank interest not included
Even small bookkeeping mistakes can affect tax liability — so correcting them quickly is important.
Step-by-Step: How to Fix a Submitted UK Tax Return
Step 1 — Check If You’re Within the Amendment Deadline
You have 12 months after the Self Assessment deadline to amend online.
If outside this period, you must write to HMRC instead (explained below).
Step 2 — Log in to Your HMRC Account
Go to your Self Assessment account and select:
- “Amend return”
- Choose the tax year
- Update incorrect figures
HMRC systems will automatically recalculate your tax.
Step 3 — Correct the Relevant Section
Typical corrections include:
- Self-employment income
- Property income
- Employment income
- Expenses
- Capital gains
- Tax relief claims
Always update supporting figures consistently (profit, tax due, payments).
Step 4 — Resubmit the Amended Return
After corrections:
✔ Review changes
✔ Check recalculated tax
✔ Submit amendment
You’ll receive confirmation from HMRC.
What Happens After You Amend a Tax Return?
There are three possible outcomes:
- You owe more tax
Pay immediately to reduce interest/penalties. - You paid too much tax
HMRC issues a refund. - No tax change
Return simply updated.
What If the Amendment Deadline Has Passed?
If more than 12 months have passed since the filing deadline, you cannot amend online.
Instead, you must write to HMRC with:
- Tax year
- UTR number
- Incorrect figures
- Correct figures
- Reason for change
This is called an overpayment relief claim or correction request.
Will You Get a Penalty for Fixing a Tax Return Mistake?
Not usually — if you correct voluntarily.
HMRC penalties depend on behaviour:
- ✔ Careless mistake: low/no penalty
- ✔ Prompt disclosure: reduced penalty
- ❌ Deliberate concealment: high penalty
Fixing errors yourself shows compliance and reduces risk.
Should You Amend a Tax Return or Contact HMRC?
Amend online if:
- Within 12-month window
- Numerical errors
- Missing income/expenses
- Wrong entries
Contact HMRC or an accountant if:
- Multiple errors
- Complex tax issue
- Capital gains mistakes
- Business accounts incorrect
- Outside amendment deadline
Special Cases: Business and Landlord Errors
Tax return mistakes are common among:
UK Landlords
Typical issues:
- Missing rental income
- Incorrect expense claims
- Mortgage interest errors
- Property sale gains
Self-Employed Businesses
Common errors:
- Incorrect profit
- Expense misclassification
- VAT adjustments missing
- CIS deductions wrong
These often require bookkeeping correction before amendment.
How to Correct Bookkeeping Errors Before Amending
Before changing your tax return, ensure records are accurate:
✔ Reconcile bank transactions
✔ Check invoices/receipts
✔ Verify expense categories
✔ Confirm income totals
✔ Review accounting software reports
Correct bookkeeping → accurate tax amendment.
When to Use an Accountant to Fix a Tax Return
You should seek professional help if:
- Tax liability changed significantly
- Multiple income sources
- Property + self-employment
- Capital gains involved
- HMRC enquiry risk
- Late amendment
An accountant ensures:
✔ Correct figures
✔ Compliance
✔ Penalty reduction
✔ HMRC communication
How to Avoid Future Tax Return Mistakes
Prevent errors with better systems:
- Maintain monthly bookkeeping
- Use accounting software
- Track income in real time
- Store digital receipts
- Review tax quarterly
- Use professional review
Good bookkeeping is the #1 way to prevent tax errors.
Key Deadlines to Remember
- 31 January — filing deadline
- 12 months after — amendment window
- Immediate — pay extra tax after amendment
Missing these increases risk of penalties.
Final Thoughts: Fixing a UK Tax Return Mistake
If you’re worried about an error in your submitted Self Assessment return, remember:
✔ Most mistakes can be corrected
✔ HMRC allows amendments for 12 months
✔ Early correction reduces penalties
✔ Accurate bookkeeping prevents issues
Taking action quickly protects you from interest, penalties, and compliance risk with HMRC.
Need help correcting a UK tax return or bookkeeping errors?
Professional support can ensure your amendment is accurate, compliant, and stress-free.
Contact Us at!
info@legacyaccountings.co.uk
+44 785 294 2435
Read Also
How to Fix Landlord Tax Return Mistake UK HMRC (Step-by-Step Guide)
