Common Self Assessment Mistakes to Avoid

Every year, HMRC receives millions of Self Assessment returns — and a significant portion contain errors. Some mistakes are innocent typos. Others can trigger investigations, penalties, or unexpected tax bills months later.

The good news is that most errors are entirely avoidable. This guide covers the most common mistakes and how to steer clear of them.

Mistake 1: Missing the Deadline

This is the most basic error, but thousands of people still make it every year.

The deadlines for 2024/25 tax year:

Return type	Deadline
Paper return	31 October 2025
Online return	31 January 2026
Payment	31 January 2026

Automatic penalties for late filing:

How late	Penalty
1 day late	£100 (even if you owe nothing)
3 months late	£10 per day (up to 90 days)
6 months late	£300 or 5% of tax due (whichever is higher)
12 months late	Another £300 or 5% (serious cases: up to 100% of tax)

How to avoid it:

• File early — you can submit from 6 April onwards
• Set calendar reminders for October (paper) or early January (online)
• Filing early doesn't mean paying early — tax is still due 31 January

Mistake 2: Forgetting Income Sources

HMRC receives information from employers, banks, and other sources. If you "forget" income, their systems will likely flag the discrepancy.

Commonly forgotten income:

• Interest from savings accounts (even small amounts)
• Dividends from shares
• Rental income from a spare room (above Rent-a-Room threshold)
• Income from side jobs or freelance work
• Cashback or referral bonuses (sometimes taxable)
• Foreign income
• State benefits (some are taxable)
• Income from previous employment in the same tax year

How to avoid it:

• Review all bank statements for the tax year
• Check for P60s or P45s from all employers
• Log into your HMRC online account — some income is pre-populated
• Keep a running list of income sources throughout the year

Mistake 3: Claiming Expenses You Can't Prove

Claiming expenses is legitimate — but you need evidence. HMRC can ask for proof up to 6 years after filing.

What HMRC expects:

Expense type	Acceptable evidence
Purchases	Receipts, invoices, bank statements
Mileage	Log book with dates, destinations, business purpose
Home office	Calculation method (simplified or actual), utility bills
Subscriptions	Invoices or payment confirmations
Professional fees	Receipts from accountant, professional body

Red flags that trigger enquiries:

• Expenses that seem high relative to income
• Round numbers (£500 exactly vs £487.32)
• Expenses that don't match the business type
• Large increases from previous years with no explanation

How to avoid it:

• Keep receipts (paper or digital) for everything you claim
• Use accounting software or a spreadsheet to track expenses
• Take photos of receipts immediately — they fade
• Be honest — if you lost a receipt, estimate conservatively

Mistake 4: Mixing Personal and Business Expenses

If you're self-employed, you can only claim expenses that are "wholly and exclusively" for business purposes. Mixed-use expenses need to be apportioned.

Examples of incorrectly claimed expenses:

• Full mobile phone bill (when used for personal calls too)
• Entire broadband bill (when family also uses it)
• Clothing that could be worn casually
• Lunch while working (not an allowable expense)
• Commuting to a regular workplace (travel to clients is fine)

How to avoid it:

• Apportion mixed-use expenses (e.g., 50% of phone bill if half business use)
• Keep a separate business bank account
• Review the "wholly and exclusively" test before claiming
• When in doubt, don't claim it

Mistake 5: Using the Wrong Figures

Simple number errors can cause significant problems.

Common numerical mistakes:

• Entering net instead of gross income (or vice versa)
• Using figures from the wrong tax year
• Transposing digits (£1,234 becomes £1,324)
• Double-counting income that appears on multiple documents
• Using calendar year instead of tax year (6 April to 5 April)

How to avoid it:

• Use a calculator and double-check additions
• Cross-reference figures against source documents
• Read HMRC's pre-populated data carefully — it may not match your records
• Take a break and review with fresh eyes before submitting

Mistake 6: Ignoring Payments on Account

First-time filers are often shocked to discover they owe more than expected because of "payments on account."

How payments on account work:

If your tax bill exceeds £1,000 (and less than 80% was collected at source through PAYE), HMRC requires advance payments towards next year's tax.

Example:

• 2024/25 tax bill: £4,000
• First payment on account (due 31 Jan 2026): £2,000
• Second payment on account (due 31 Jul 2026): £2,000
• Total due in first year: £8,000

How to avoid surprises:

• Understand that payments on account are normal, not extra penalties
• Budget for 150% of your tax bill in the first year
• You can reduce payments on account if you expect lower income next year (but penalties apply if you reduce too much)

Mistake 7: Wrong Bank Details

If you're due a refund and enter incorrect bank details, your money goes to the wrong account.

What happens:

• HMRC sends the refund to the account you specified
• Recovering money from wrong accounts is difficult and slow
• You may need to wait months for resolution

How to avoid it:

• Triple-check your sort code and account number
• Use copy-paste from your banking app if possible
• Verify the account holder name matches exactly

Mistake 8: Not Declaring All Self-Employment

If you have multiple self-employed activities, each should be reported.

Examples:

• Freelance design work AND selling crafts online
• Consulting AND rental income
• Multiple trades or businesses

How to avoid it:

• Report each self-employment separately on the SA103 supplementary pages
• Don't lump different businesses together
• Each trade has its own income and expenses section

Mistake 9: Forgetting Student Loan Repayments

If you have a student loan and are self-employed, repayments are calculated through Self Assessment — not automatically deducted.

Current thresholds (2024/25):

Plan type	Annual threshold	Rate
Plan 1	£24,990	9%
Plan 2	£27,295	9%
Plan 4 (Scotland)	£31,395	9%
Plan 5	£25,000	9%
Postgraduate Loan	£21,000	6%

How to avoid issues:

• Select the correct loan plan type on your return
• The system calculates repayments automatically
• Check the calculation matches what you expect

Mistake 10: Submitting Without Review

The excitement of finishing often leads people to submit immediately without checking.

Review checklist:

• All income sources included
• Expense figures match your records
• Personal details are correct
• Bank details are accurate
• No warning messages or prompts ignored
• Calculation seems reasonable

How to avoid it:

• Save and close the return
• Come back the next day with fresh eyes
• Review the tax calculation — does it seem right?
• Ask someone else to check if possible

What to Do If You've Made a Mistake

If you spot an error after submitting:

Within 12 months of the deadline:

• Log in to HMRC online
• Select "Correct your return"
• Make the necessary changes and resubmit

After 12 months:

• Write to HMRC explaining the error
• Include all relevant details and correct figures
• HMRC will recalculate and issue a revised statement

If HMRC finds the error:

• You may receive a "nudge letter" asking you to check your return
• Respond promptly and honestly
• Penalties may apply, but voluntary disclosure usually results in lower penalties

Penalties for Errors

HMRC applies penalties based on the nature of the error:

Error type	Penalty range
Careless mistake	0-30% of extra tax due
Deliberate error	20-70% of extra tax due
Deliberate and concealed	30-100% of extra tax due

Voluntary disclosure before HMRC discovers the error reduces penalties significantly.

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This guide is for informational purposes only and does not constitute tax, legal, or financial advice. Tax rules change frequently. Always verify current requirements on GOV.UK or consult a qualified accountant for your specific situation.

Official Sources

• Self Assessment penalties - GOV.UK
• Keeping your pay and tax records - GOV.UK
• Correct errors in your tax return - GOV.UK
• Expenses if you're self-employed - GOV.UK