Six Common Self Assessment Mistakes (and How to Avoid Them)

Filing a Self Assessment tax return is a familiar annual task for millions of UK taxpayers, and one that often comes down to the wire. In the 2021/22 tax year, over 11.7 million people filed a Self Assessment return. Around 800,000 submitted on the final day, with tens of thousands filing in the final hour before the 31 January deadline.

With that level of pressure, mistakes are common. Some errors are minor and easy to fix. Others can result in:

• Unexpected tax bills
• Late payment penalties
• Interest charges
• In serious cases, HMRC investigations

Understanding the most common Self Assessment mistakes, and how to avoid them, can help you file confidently and avoid unnecessary stress.

Entering an incorrect UTR or National Insurance number

Your Unique Taxpayer Reference (UTR) and National Insurance (NI) number are essential identifiers used by HMRC to link your return to your tax record. Entering either incorrectly can delay processing, trigger follow-up queries from HMRC, or cause payments to be allocated incorrectly

You can find these details on:

• Your HMRC Personal Tax Account
• Previous Self Assessment returns
• Letters or emails from HMRC

Tip: Double-check these numbers carefully before submitting, especially if you’re filing manually or switching software.

Failing to declare all taxable income

One of the most common, and most costly, mistakes is not reporting all taxable income. Your Self Assessment return must include all relevant income sources, including:

• Employment income (UK or overseas)
• Self-employed income
• Tips and commission
• Rental income (UK and overseas)
• Bank and savings interest
• Dividend income
• Pension income (UK and overseas)
• Certain state benefits (e.g. maternity allowance)
• Capital gains from selling taxable assets

Missing income can lead to underpaid tax and potential penalties. If you’re unsure whether something counts as taxable income, HMRC provides guidance and support.

👉 Contact HMRC for Self Assessment help

Forgetting to include the right supplementary pages

The main Self Assessment form (SA100) often isn’t enough on its own. Depending on your circumstances, you may also need supplementary pages, such as:

• SA103 – self-employed income
• SA105 – UK property income
• SA108 – capital gains
• SA101 – additional information

Failing to include the correct pages can result in missing income declarations, incorrect tax calculations, and follow-up requests from HMRC. You can find the full, up-to-date list on GOV.UK.

👉 Self Assessment supplementary pages

Not claiming all allowable expenses

Many self-employed people overpay tax simply because they forget to claim allowable expenses. Allowable expenses are legitimate business costs that can be deducted before tax is calculated, such as:

• Office costs
• Travel expenses
• Professional fees
• Equipment and software
• Use of home for business

Failing to claim these means paying more Income Tax and National Insurance than necessary. Expenses should be recorded on the SA103 self-employment pages and supported by accurate records.

👉 HMRC guidance: Allowable expenses if you’re self-employed

Ticking the wrong boxes

Self Assessment forms include a large number of tick boxes — and it’s easy to select the wrong option if you’re rushing or unsure. Common issues include:

• Incorrect residency status
• Missing declarations
• Selecting reliefs that don’t apply

These errors can change how your tax is calculated, trigger HMRC queries, and delay refunds

Tip: Read the guidance notes carefully and consider using HMRC-recognised software, which can flag inconsistencies and reduce the risk of mistakes.

Making pension contribution errors

Pensions are another area where errors frequently occur, particularly for self-employed taxpayers and higher earners. On the SA100 tax return, pension contributions are split across several sections:

• Payments to registered pension schemes: Enter the gross amount of personal contributions where basic-rate relief is claimed by the provider
• Page 4 (SA100): Boxes 1–3: UK pension contributions. Box 4: Overseas pension contributions
• Page 3 (SA100): Boxes 8–12: UK pensions and annuities received (including lump sums and State Pension)

Mixing up contributions made, pension income received, and net vs gross amounts can lead to incorrect tax relief or unexpected tax bills.

How to correct a Self Assessment mistake

If you spot an error after submitting your return, you usually have 12 months from the filing deadline to amend it. For example a mistake in a 2024/25 return (deadline 31 January 2026) can typically be corrected until 31 January 2027. Depending on the correction, you may:

• Owe additional tax
• Be due a refund

Fixing errors promptly can help reduce penalties and interest.

Take your time, it pays off

Self Assessment doesn’t need to be rushed — and taking extra care can save you money, time and stress. By:

• Checking your details
• Declaring all income
• Claiming the reliefs you’re entitled to

…you reduce the risk of errors and ensure you’re paying the right amount of tax – no more, no less.