Self Assessment tax return guide: what you need to know

Who may need to complete a Self Assessment tax return?

You may need to send a Self Assessment tax return if you are self-employed, a partner in a business partnership, have certain types of untaxed income or have other circumstances that require you to report income to HMRC.

For example, HMRC currently states that a sole trader generally needs to send a tax return if their gross trading income was more than £1,000 in the tax year. Other reasons can include certain property income, savings interest, dividends, foreign income, Capital Gains Tax liabilities or the High Income Child Benefit Charge.

The rules can depend on your individual circumstances, so if you are unsure whether you need to file, use HMRC’s online checker or seek professional advice.

What information do you need for your tax return?

Before starting your return, gather the records and documents you need to report your income and claim any relevant reliefs or allowable expenses.

Depending on your circumstances, this could include business income and expense records, employment income, pension information, savings interest, dividends, property income, charitable donations and details of any other taxable income.

If you are self-employed, keeping accurate records of your business income and expenses throughout the year can make completing the return much easier. HMRC allows records to be kept on paper, digitally or using bookkeeping software.

Keep your records organised

Good record keeping is an important part of completing Self Assessment accurately. Your records should support the figures you include on your tax return and may need to be produced if HMRC checks your return.

For a self-employed business, this can include records of sales, business expenses, bank transactions and other information needed to calculate your taxable profit.

Keeping records throughout the year is generally easier than trying to reconstruct everything shortly before the filing deadline. A separate business bank account and regular bookkeeping can also make it easier to identify business transactions.

Important Self Assessment deadlines

The UK tax year runs from 6 April to 5 April. You can normally submit an online Self Assessment return from 6 April following the end of the tax year.

For the 2025/26 tax year, the online Self Assessment filing deadline is 31 January 2027. The tax due for that tax year is also generally payable by 31 January 2027.

If you are filing a paper return for the 2025/26 tax year, HMRC must receive it by 31 October 2026. If you are registering for Self Assessment for the first time, there is also a registration deadline that can apply before the filing deadline.

It is worth checking the current HMRC deadlines for your circumstances rather than relying on dates from an earlier tax year.

What are payments on account?

Some people who complete Self Assessment have to make payments on account towards the following year’s tax bill. These are advance payments towards your tax liability rather than an additional tax charge.

Where payments on account are required, they are normally made in two instalments, with the first due by 31 January and the second by 31 July. Each payment is normally based on 50% of the relevant previous tax liability, subject to the rules that apply to your circumstances.

Payments on account can make the first Self Assessment bill higher than expected because you may need to pay both a balancing payment for the year just ended and an advance payment towards the next year.

If your circumstances have changed and you expect your tax liability to be lower, there are circumstances in which you can ask HMRC to reduce your payments on account. This should be considered carefully because reducing them too far can result in tax and interest becoming payable later.

How is the tax return submitted?

Most individuals who complete Self Assessment can submit their return online through HMRC’s online service. You will need to sign in and, where applicable, have your Unique Taxpayer Reference (UTR) available.

Some types of return or circumstances require different forms or commercial software. For example, partnerships and certain specialist situations may not be covered by HMRC’s standard online filing service.

You do not have to complete an online return in one sitting. HMRC’s online service allows you to save your progress and return to it later.

Common mistakes to avoid

One of the most common problems is leaving the return until close to the deadline. Rushing can make it easier to miss income, overlook allowable expenses or enter figures incorrectly.

Other issues can arise when people forget about additional sources of income, use incomplete business records or misunderstand the difference between accounting figures and the figures that need to be reported for tax purposes.

If you have income from more than one source, have recently started a business, receive property or foreign income, or have other more complicated tax affairs, make sure you understand which sections of the return apply to you.

What happens if you make a mistake?

If you discover an error after submitting your tax return, you may be able to amend it. HMRC generally allows an online Self Assessment return to be corrected within 12 months of the filing deadline.

An amendment can change the amount of tax you owe or the amount you are due to receive. If an amendment means that more tax is payable, you should check the updated payment position and any effect on payments on account.

If you are unsure how an error should be corrected, particularly where it affects several tax years or involves a significant amount, professional advice can help you understand the appropriate process.

When might an accountant help?

Completing a straightforward Self Assessment return can be manageable if your tax affairs are simple and your records are well organised. However, professional support can be useful when your circumstances are more complicated or you want help understanding your tax position.

An accountant can help prepare and review your return, identify relevant allowable expenses and reliefs, explain your tax calculation and help you plan for upcoming tax payments.

Professional support can also be useful if you are self-employed and need help keeping your bookkeeping and tax records organised throughout the year rather than only at Self Assessment time.

A simple checklist before submitting your return

Before submitting your Self Assessment return, check that you have included all relevant sources of income and that your business records support the figures you have entered.

Make sure you have considered relevant employment, self-employment, property, savings, dividend, pension and foreign income where applicable. Check any expenses or tax relief claims carefully and make sure you understand the amount shown as payable.

Finally, check the filing and payment deadlines that apply to you. Submitting your return early can give you more time to identify and correct problems and can help you budget for the tax bill.