Making a mistake on your Self Assessment tax return is easier than you might think — and HMRC does not always let it slide. This guide covers the most common errors UK taxpayers make, the questions our clients ask us most often, and exactly how to put things right before they become a problem.
What is the Self Assessment tax return deadline in the UK?
Missing a deadline is one of the most avoidable — and most costly — mistakes you can make. The date you need depends on how you file:
- Paper return: 31st October 2026 (for the 2025/26 tax year)
- Online return: 31st January 2027 (for the 2025/26 tax year)
Most people file online, which is quicker, more secure, and gives you an extra three months compared to paper. Submit even one day late and HMRC charges you automatically — there is no grace period.
| How Late | Penalty |
|---|---|
| 1 day late | £100 automatic fine |
| 3 months late | £10 per day (up to 90 days = up to £900) |
| 6 months late | 5% of tax owed or £300 — whichever is higher |
| 12 months late | A further 5% or £300 — whichever is higher |
Tip: Set calendar reminders for both October and January. Better still, aim to file in November or December — that way you know exactly what your bill is before it falls due on 31st January.
What are Payments on Account and why is my HMRC bill higher than expected?
If your tax bill for the year comes to more than £1,000, HMRC will ask you to make advance payments towards the following year’s bill. These are called Payments on Account. Each payment is half of your previous year’s bill, due in January and July.
For people filing Self Assessment for the first time, this is often a genuine shock — your January bill can appear to have nearly doubled because you are paying last year’s tax and a chunk of next year’s at the same time.
Example: You owe £2,400 for 2024/25. On 31st January you also owe a first Payment on Account of £1,200 for 2025/26 — a combined total of £3,600 in one payment.
If your income is going to be lower next year, you can apply to reduce your Payments on Account — but if you reduce them by too much, HMRC will charge interest on the shortfall.
I have lost my UTR number, how do I find it?
Your Unique Taxpayer Reference (UTR) is a 10-digit number HMRC uses to identify your tax records. You need it every time you file a Self Assessment return.
If you have misplaced it, you can find it in several places:
- Any previous tax returns or letters from HMRC
- Your HMRC Personal Tax Account online
- Your SA302 tax calculation document
- By calling the HMRC Self Assessment helpline: 0300 200 3310
If you are a limited company director, note that your personal UTR is different from your company’s Corporation Tax UTR — make sure you are using the correct one when filing your personal return.
What expenses can I claim on my Self Assessment return?
You pay tax on your profit, not your total income. Every allowable expense you claim reduces your profit — and therefore your tax bill. A surprising number of self-employed people under-claim and end up paying more tax than they need to.
Common allowable expenses for the self-employed
- Office costs — stationery, software subscriptions, printer ink
- Business travel — fuel, train fares, parking (not commuting to a regular workplace)
- Uniforms and protective clothing — not everyday clothing, even if worn exclusively for work
- Staff costs if you employ people
- Marketing, advertising and website costs
- Professional fees — accountant, solicitor, business insurance
- Bank charges on a business account
- Use of home as office flat rate of £6 per week, or a calculated proportion of bills
- Training directly related to your current trade or business
- Phone and broadband — the business-use portion only
Common mistake: Claiming for everyday clothing because you only wear it for work. HMRC requires it to be a recognisable uniform or protective gear a suit or smart dress does not qualify.
What if I do not have all my records, can I still file my tax return?
Good record-keeping is the foundation of an accurate tax return. But if paperwork has gone missing, you can still file using provisional figures rather than miss the deadline. Once the correct figures are available you can amend your return — you have up to 12 months after the submission deadline to do so.
Do not forget to sign a paper tax return
If you file on paper rather than online, do not forget to sign and date the form before posting it. HMRC will not process an unsigned return, and a photocopy signature is not accepted. It sounds obvious, but it catches people out every single year.
Looking ahead: From April 2026, Making Tax Digital for Income Tax (MTD IT) applies to sole traders and landlords earning over £50,000 annually. This requires quarterly digital updates to HMRC, which makes real-time record-keeping essential rather than optional.
What happens if I put the wrong figures on my tax return?
Typos happen to everyone. HMRC generally will not issue a penalty if you have taken reasonable care when completing your return — but entering incorrect figures that lead to underpaid tax can result in interest charges from the original due date.
When you file online, the system handles most calculations for you. However, you still need to verify that every figure matches your source documents — P60, P11D, bank statements, invoices. Transposing even two digits (entering £4,500 as £5,400, for example) can create an unexpected bill.
Even if you use an accountant, the legal responsibility for the accuracy of your return rests with you. Always review the completed return carefully before it is submitted.
Do I need to declare pension contributions on my Self Assessment?
Yes , and it works in your favour. Any private pension contributions made during the tax year need to be declared on your return, and doing so lets you claim tax relief on those payments.
Basic rate taxpayers automatically get 20% relief added to their pension pot by the provider. Higher rate (40%) and additional rate (45%) taxpayers need to claim the extra relief through Self Assessment and many miss it entirely.
For 2025/26, the annual pension allowance is £60,000 (or 100% of your earnings if lower). This is one of the most valuable tax planning opportunities available to self-employed people.
What supplementary pages do I need to include with my tax return?
The main SA100 form does not cover everything. If you have additional income sources or specific claims, you need to attach the correct supplementary pages. Leaving them out means HMRC does not have the full picture and could raise an enquiry.
When do you need supplementary pages?
- Rental or property income — SA105
- Self-employment income — SA103
- Employment income if you are also employed — SA102
- Foreign income or overseas employment — SA106
- Capital gains from property or investments — SA108
- Income from share schemes or dividends
- Life insurance policy pay-outs
- Taxable benefits in kind from an employer
- Pension lump sums from overseas schemes
- Loss relief claims or post-cessation receipts
If you receive income from multiple sources — freelance work, a rental property, dividends from a limited company — it is easy to miss a required supplementary page. An accountant can make sure nothing is overlooked.
How long do I need to keep my tax records in the UK?
HMRC can open an enquiry into past returns, so holding onto your records is not optional. The rules depend on your situation:
- Self-employed or sole trader: 5 years after the 31st January submission deadline for that tax year
- Employee with no other income: 22 months after the end of the relevant tax year
- Limited company director: At least 6 years
Cloud-based bookkeeping software makes this much simpler — receipts can be photographed and stored digitally, income and expenses are logged automatically, and everything is in one place if HMRC ever asks questions.
Can I correct a mistake on my Self Assessment tax return after submitting it?
Yes. You can amend a submitted return up to 12 months after the original filing deadline — not 12 months from the date you submitted it. This is known as making an amendment.
For the 2025/26 tax year:
- Online filing deadline: 31st January 2027
- Last date to amend that return: 31st January 2028
If the error meant you paid too little tax, HMRC may charge interest from the original due date — so act as soon as you spot the mistake rather than waiting.
To amend online: log into your HMRC account, go to your submitted return, and select Amend return. If you filed on paper, you will need to write to HMRC with the corrected figures and a brief explanation.
If you discover a more serious error going back more than 12 months, HMRC has a voluntary disclosure process. Getting ahead of it is always better than waiting for them to find it themselves.
How can I reduce my tax bill before my next Self Assessment return?
Tax bills can be reduced considering following ways.
Consider using a Stocks and Shares ISA for investments
UK residents can save or invest up to £20,000 per year in an ISA. Any returns inside an ISA are completely free of Income Tax and Capital Gains Tax — and you do not need to declare them on your Self Assessment return at all, unlike regular savings or investment accounts where income above certain thresholds is taxable.
Increase your pension contributions
Pension contributions reduce your taxable profit. The more you contribute up to the annual allowance, the less tax you pay now while building a fund you can access from age 57 (from 2028). Self-employed people do not receive employer contributions, which makes maximising their own even more important. A Self-Invested Personal Pension (SIPP) gives you flexibility over where the money is invested.
Consolidate old pensions and savings
If you have pension pots scattered across old employers or providers, bringing them together can reduce admin at tax time and may improve investment returns. Always check for exit fees or protected benefits before transferring, and speak to a regulated financial adviser before making any decisions.
Note: The information in this blog is intended for general awareness only and should not be taken as definitive tax advice. Rules can change and individual circumstances vary. If you need help with your Self Assessment or tax return, speak to a qualified accountant.
