How to fill in a self-assessment tax return

What is a self-assessment tax return?

A self-assessment tax return is an online or paper form that has to be submitted to HMRC every year by those who owe tax on income they've received.

In some cases, tax is deducted automatically from your wages or pension – known as PAYE.

However, if you receive any other income – such as from self-employment, property, capital gains, or dividends – you need to report this to HMRC by sending a self-assessment tax return.

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Who pays self-assessment tax?

More than 11.5m people filed a self-assessment tax return for the 2023-24 tax year.

If you're self-employed, you'll need to submit a self-assessment tax return every year if you earn more than £1,000, to pay income tax and National Insurance on your profits.

Other people who may need to fill in a tax return include anyone who:

• earned a taxable income of £150,000 or more in the last tax year as an employee or pensioner
• earned £10,000 or more from savings interest, or investment income (however, note that you should also declare income from savings interest above the personal savings allowance, and dividend income above the dividend allowance)
• earned £2,500 or more in untaxed income – for instance, from tips or commission
• owes capital gains tax from selling assets at a profit
• claims child benefit, if you or your partner's income is over the High Income Benefit Charge threshold
• receives taxable income from abroad, or lives abroad but receives an income in the UK
• receives state pension payments that exceed your personal allowance and it's your only source of income
• is a business partner, or director of a limited company
• is a trustee of a registered pension scheme or other trust
• is a trustee or representative of someone who has died
• is a 'name' at the Lloyd's of London insurance market
• is a minister of religion
• received a P800 form from HMRC saying you didn't pay enough tax last year, and you haven't yet paid the outstanding sum.

In some cases, you may need to complete a self-assessment tax return and also pay via PAYE: for instance, if you receive a private pension or investment income, make a taxable capital gain or run a business on the side of your employment.

If you run a limited company, you'll need to file a company tax return in addition to a tax return on your personal income.

Will I be sent a tax return?

You'll usually be sent a tax return if:

• you have untaxed income from investment, land or property, or from overseas.
• you make capital gains above the annual exempt amount (£3,000 in 2025-26 and 2024-25).
• you were required to fill in a tax return last year.
• you're a pensioner who gets reduced age-related allowance - though you may be sent a special short version that requires fewer details.

You shouldn't rely on HMRC to contact you before submitting a tax return if you know you owe tax. It's your responsibility to make sure you declare all taxable income each year.

If you receive a tax return, you must return it, regardless of whether you owe tax or not.

How to register for self-assessment

If you're looking to submit a tax return for the first time, you'll need to register for self-assessment first.

Here are the steps you need to take:

1. Register with HMRC: The process will vary depending on whether you're self-employed, registering a partnership or not self-employed - you should click on the option that applies to you. You can register online via HMRC.
2. Get your Unique Taxpayer Reference (UTR) number: HMRC will send this to you in a letter after you register. The letter will give instructions on how to set up your Government Gateway account.
3. Use your activation code for your Government Gateway account: Once this is done, you'll be sent another letter in the post containing your activation code. You'll need this to complete the set-up of your account - you should do this promptly as the code will expire.
4. Complete your account setup: It's only once your Government Gateway account is up and running that you'll be able to log in and submit your tax return.

HMRC warns that the whole process could take up to 20 working days, so make sure you don't leave it until the last minute.

How to fill in a self-assessment tax return

When you submit your tax return online, you'll just need to fill out the sections that apply to you. We explain the process in our guide to online tax returns.

For paper tax returns, you'll need to work out which sections are relevant. Most people will just have to fill out the SA100 form. However, several supplementary pages may apply to your circumstances. We explain more in our guide to paper tax returns.

Some employees, pensioners and self-employed people with a turnover of under £90,000 can be sent a simplified SA200 return. At four pages long, it's much shorter. Unfortunately, you can't opt to fill in this shorter form – HMRC will decide and send it out to you.

Details you may need to include on your tax return include:

• Income: all taxed and un-taxed income from self-employment, taxable interest from savings, dividends from shares, or capital gains from selling assets
• State pension: the total amount of state pension payments you were entitled to receive, plus any lump sum
• Private pensions: detail the gross amount of any annuities or lump sums
• Benefits: include anything you've received in incapacity benefit and jobseeker's allowance, plus the total of taxable benefits from bereavement allowance, carer's allowance or industrial death benefit
• Other income: this is anything not related to interest or dividends, and you can also include any allowable expenses related to this income
• Pension contributions: all payments where deductions were made after tax
• Charitable donations: include the total amount of Gift Aid donations
• Blind Person's Allowance: you just need to confirm whether or not you're claiming this
• Student loan repayments: detail deductions made by your employer
• High Income Child Benefit Charge: this is only for those receiving child benefit when they, or their partner, earn more than £60,000 in 2024-25 or 2025-26.
• Marriage allowance: the marriage allowance means you can transfer some of your personal allowance to your spouse if your income is less than the personal allowance (£12,570 in 2024-25 and in 2025-26).

Before you start filling out your tax return, it's best to gather all of the information you'll need.

Make sure you have records of:

• Your National Insurance number.
• Your UTR number.
• Accounts, invoices, receipts or other records of income.
• Records of any relevant expenses.
• Contributions to pensions or charities
• P60 and P45 forms.

Depending on your circumstances, you might need extra records, such as tenancy agreements.

Self-assessment tax return deadlines

Self-assessment tax is based on your income from the last tax year - not on the calendar year.

The tax year runs from 6 April to 5 April, and your tax return will be due the following January.

The self-assessment deadlines for your 2024-25 return are as follows:

• 5 October 2025: deadline to register for self-assessment for the first time
• 31 October 2025: paper tax return deadline
• 31 January 2026: online tax return deadline
• 31 January 2026: tax payment deadline for 2024-25 tax owed. If you pay your tax by payments on account you may have already made payments towards this bill.

HMRC has the power to charge increasingly expensive penalties if you miss the tax return deadline, which starts with a £100 fine from the first day your return is late.

If you don't pay your tax by 31 January 2026, HMRC will charge you interest, set at the Bank of England base rate plus 4%.

• Find out more: late tax returns and penalties for mistakes

How do self-employed tax returns differ?

If you are self-employed, you’ll need to detail all income you've received during the tax year in question. This can be offset against any expenses and allowances you qualify for, meaning that your resulting tax bill will only be based on your taxable profits.

Here are three key things self-employed taxpayers need to be aware of when completing their returns:

1. Tax period

While HMRC issues deadlines to submit your return and pay the tax you owe, you can choose the dates that your tax is calculated for. This is your 'accounting period', which usually spans 12 months.

Many sole traders choose between 31 March to 5 April for their year end, as the tax year finishes on 5 April and HMRC says that accounts prepared to 31 March will count as being prepared to the end of the tax year.

This means your accounts will start again in accordance with the start of a new tax year, and you'll have the longest possible time in which to pay the tax you owe, which won't be due until 31 January the following calendar year.

Your 'basis period' is the period HMRC assesses your tax on. From April 2024 all unincorporated businesses have been required to use 6 April to 5 April as their basis period, regardless of their accounting period.

The 2023-24 tax year was used as a transitional year where businesses were expected to calculate a basis period that's longer than 12 months if their accounting period doesn't match up to the tax year.

Many businesses may now find it easier to move their accounting period to 6 April to 5 April.

2. Payment schedule

Calculating what tax is due when can be tricky to get your head around at first. Here is a breakdown of how self-employed tax payments work:

Year 1: in your first year, you're taxed on profits made from the date you started your business to the end of the tax year. For instance, if you started your business on 5 February, you'd be taxed on the profits you made between 5 February and 5 April.

Year 2: in the second year, you're taxed on profits for 12 months to your 'accounting date'. In our example, this would again be 5 February - so you'll be charged on profits made between 5 February to 4 February, effectively paying tax for a second time on profits between 5 February and 5 April. This is referred to as 'overlap profits' and can be claimed back - but only when you cease trading.

Year 3: your tax bill will be based on profits made to your accounting year.

3. Payments on account

After your first full year of business, as well as paying tax for the tax year that's just ended, you are also required to pay tax for the current year in two instalments - the first on 31 January and the second on 31 July. 

These are known as 'payments on account’ and are based on what you owed the previous tax year.

In this Q&A, we tell you everything you need to know about making payments on account.

Which tax return forms do you fill in for self-employed income?

It's important to make sure you declare all relevant income on your tax return; most income sources are given different sections if you file online.

If you file a paper tax return, you'll need to fill out supplementary pages.

The main self-assessment section taxpayers need to fill out is SA100 and it's the first section you're sent to if you file your return online.

If you're self-employed, you'll also need to fill out the section SA103S (the short version) or SA103F (the full version).

You can fill in the short one if your turnover for the 2024-25 tax year was £85,000 or less (rising to £90,000 for 2025-26) and you have no complications, such as a change of accounting date.

However, you can't use the short form if your accounting period - the dates you choose to prepare your accounts for each year - isn't the same as your basis period (ie your business year, which is the period HMRC assesses your tax on).

This means many people won't be able to use the short supplement in their first year of trading.

It's up to you to make sure you use the right supplement, so if you opt for the short form make sure there are no complications that could invalidate it.

• Useful link: check HMRC's notes to help you fill in the supplement forms - the short form and the full form.

What if I'm a sole trader with a limited company or partnership?

In addition to the supplements already covered, if you're in partnership, you need the partnership supplement (SA104), which also has a full and a short version.

If you're the director of a limited company, you will count as an employee of the business for tax purposes, and will therefore need to fill out the employment supplement (SA102).

How to keep your records

Regardless of whether you opt to file your tax return using the paper forms or online, you must keep the records and documents that give evidence of the information you've stated in your return.

This could include things like bank statements, receipts and contracts - anything that documents your income and business dealings.

You must keep this evidence because HMRC has the right to request to see them if it carries out an investigation into your tax liabilities - and having everything to hand could save you from paying a penalty if HMRC believes you've made a mistake on your tax return.

Check our Q&A below for anything you want to know about self-employment tax and record keeping.

Submit your tax return

You can tackle your 2024-25 tax return with the tax calculator service from GoSimpleTax. It can help you to tot up your tax bill, get tips on where to save and submit your return directly to HMRC.

Tax returns if your circumstances change

If your circumstances change, and you start earning untaxed income, you must let HMRC know by 5 October following the end of the tax year. It will then decide whether you need to complete a tax return.

If you used to send a tax return but don't need to anymore (for instance, if you're no longer self-employed), contact HMRC to close your self-assessment account.

Help with self-assessment

The UK tax system can be difficult to navigate, so we've rounded up commonly asked questions about filing your self-assessment tax return.