How to file a Self Assessment tax return online

Published Tuesday 26 May 2026

Key takeaways

• You must file a Self Assessment tax return if you're self-employed, earn untaxed income from property or investments, or receive more than £150,000 through PAYE. Register by 5 October after the end of the tax year you need to file for.
• The online filing deadline is 31 January following the end of the tax year, giving you three extra months compared to the paper deadline of 31 October. Missing the deadline triggers an immediate £100 penalty, with further charges if you're more than three months late.
• Gather your Unique Taxpayer Reference (UTR), National Insurance number, income records, and expense documentation before you start. Having everything ready helps you file accurately and avoid delays.
• Making Tax Digital for Income Tax starts in April 2026 for sole traders and landlords earning over £50,000. You'll need compatible software to submit quarterly updates instead of a single annual return.

Do I need to file a Self Assessment tax return?

You must file a Self Assessment tax return if HM Revenue and Customs (HMRC) requires you to report untaxed income. This applies to anyone who's self-employed, but it can also apply to employees and other individuals with certain types of income.

If you're self-employed as a sole trader or in a partnership, you're responsible for calculating and paying your own income tax through Self Assessment. You need to file even if you also have a regular job where tax is deducted through Pay As You Earn (PAYE).

Employees and other individuals may also need to file if they have:

• rental income from property
• dividend income above the £500 tax-free allowance
• savings interest above your personal savings allowance
• total employment income over £150,000 through PAYE
• income from multiple jobs where tax wasn't properly deducted
• capital gains that need to be reported

If you're not sure whether you need to file, you can check if you need to send a tax return on the GOV.UK website.

How to register for Self Assessment

Before you can file a tax return, you need to register with HMRC for Self Assessment. The process takes a few weeks, so it's worth starting early.

Follow these steps to set up your account and get your tax reference number.

1. Complete the online registration form

Go to the HMRC registration service on GOV.UK and follow the steps for your situation, whether you're self-employed, registering a partnership, or have other untaxed income.

2. Receive your Unique Taxpayer Reference

HMRC will send your 10-digit UTR by post. This usually takes around 10 working days.

3. Set up your Government Gateway account

Use the activation code sent with your UTR to create your Government Gateway account. You'll use this to access HMRC's online services.

4. Access online filing

Log in to the Government Gateway to start, complete, and submit your tax return.

Allow up to 20 working days from registration to receiving your UTR and activation code. If you're registering close to a filing deadline, factor this time into your plans.

What are the Self Assessment deadlines?

Filing your Self Assessment on time helps you avoid penalties from HMRC. There are several key dates throughout the year to keep track of.

The main Self Assessment deadlines are:

• Tax year period: 6 April to 5 April the following year
• Registration deadline: 5 October after the end of the tax year you need to file for
• Paper filing deadline: 31 October
• Online filing deadline: 31 January at midnight (recommended)
• Tax payment deadline: 31 January
• Second payment on account: 31 July

Filing online gives you three extra months compared to the paper deadline, plus automatic calculations that reduce errors and instant confirmation of your submission.

From April 2026, Making Tax Digital (MTD) for Income Tax will introduce quarterly reporting deadlines for sole traders and landlords earning over £50,000 a year. Instead of one annual return, you'll submit updates by 7 August, 7 November, 7 February, and 7 May, followed by a final declaration. Those earning over £30,000 will join from April 2027, and those earning over £20,000 from April 2028.

What are the penalties for missing Self Assessment deadlines?

HMRC charges penalties if you file your tax return late or pay your tax bill after the deadline. Understanding how penalties work can help you avoid unnecessary charges.

Late filing penalties build up over time:

• One day late: £100 immediate penalty, even if you don't owe any tax or have already paid what you owe
• Three months late: £10 per day for up to 90 days, on top of the initial £100 penalty (maximum additional charge of £900)
• Six months late: 5% of the tax you owe, or £300, whichever is higher
• 12 months late: a further 5% of the tax you owe, or £300, whichever is higher

Late payment penalties also apply if you don't pay your tax by 31 January. HMRC charges 5% of the unpaid tax at 30 days, six months, and 12 months after the payment deadline. Interest is also added to any outstanding amount from the date it was due.

If you're struggling to pay on time, contact HMRC before the deadline. You may be able to set up a Time to Pay arrangement to spread the cost.

What information is needed to fill in a Self Assessment tax return?

Gathering the right documents before you start helps you avoid delays and file accurately. The documents fall into three categories: identification, income records, and tax relief details.

For tax identification, have your 10-digit Unique Taxpayer Reference (UTR) and your National Insurance number to hand.

You'll also need your income records:

• details of untaxed income from self-employment, property, and savings
• P60 or P45 forms if you've been employed during the tax year
• P11D forms showing any benefits in kind from your employer
• records of business expenses you plan to deduct

If you're claiming tax relief, gather your pension contribution details and any charitable donation records for Gift Aid claims.

Keeping organised records throughout the year makes this step much quicker when the filing deadline approaches.

How to access your Self Assessment online

To file online, you'll need to sign in to HMRC's service through the Government Gateway. The process only takes a few minutes if you have your Government Gateway credentials to hand.

1. Go to the HMRC login page

Visit the HMRC Self Assessment login page and enter your Government Gateway user ID and password.

2. Verify your identity

If this is your first time signing in, you may need to prove your identity. Have a photo ID ready, such as a UK passport or driving licence.

3. Start your return

Once logged in, you can start your return, save your progress, and submit when you're ready.

How to complete the SA100 form

The SA100 is the main Self Assessment tax return form. It covers your personal details, income, tax reliefs, and any other information HMRC needs to work out your tax bill.

The SA100 includes sections for:

• dividends and interest from savings and investments
• pension income and annuities
• student loan repayments
• charitable donations for Gift Aid claims
• tax allowances and reliefs you're claiming

You only need to fill in the sections that apply to you. HMRC's online system will guide you through each one and calculate your tax automatically.

Depending on your circumstances, you may also need supplementary pages to report specific types of income:

• SA102: employment income (employees and company directors)
• SA103S or SA103F: self-employment income (short or full versions)
• SA104S or SA104F: partnership income (short or full versions)
• SA105: UK property income
• SA106: foreign income or gains
• SA108: capital gains
• SA109: non-UK residents or dual residents

Check your return carefully before you submit it. After submission, HMRC will confirm how much tax you owe and when it's due.

Filing self-employment taxes for the first time

If you've recently become self-employed, you'll need to register for Self Assessment before you can file your first tax return. The process is straightforward, but there are a few deadlines to be aware of.

Register with HMRC by 5 October after the end of the tax year you need to file for. For example, if you started working for yourself during the 2025/26 tax year (6 April 2025 to 5 April 2026), you'd need to register by 5 October 2026. Missing this deadline could result in a penalty.

Your first Self Assessment tax bill may be higher than you expect. That's because HMRC may ask you to make payments on account for the following year at the same time as paying the tax you owe for your first year. This can mean paying up to 150% of your first year's tax bill in one go.

Can my accountant or bookkeeper file my tax return?

Yes. If you'd prefer professional help, an accountant or bookkeeper can prepare and file your Self Assessment on your behalf. This can be especially useful if you have complex finances, multiple income sources, or simply don't have the time.

Several types of professionals can help:

• Qualified accountants: can handle all aspects of your Self Assessment
• Registered tax agents: specialise in tax return preparation and submission
• Bookkeepers: offer Self Assessment services alongside day-to-day record-keeping

To appoint someone to act on your behalf, use HMRC's online authorisation service. Your accountant or bookkeeper will guide you through this process.

Your professional will need the same information you'd use to complete the return yourself. That includes your UTR, National Insurance number, income records from all sources, expense documentation, and any records for tax reliefs you're claiming. Keeping organised records throughout the year and sharing them in good time will help your accountant or bookkeeper file accurately and on schedule.

Can I pay my Self Assessment tax in advance?

In some cases, HMRC requires you to make advance payments toward your next tax bill. These are called payments on account, and they apply if your previous tax bill was £1,000 or more.

Payments on account are paid in two instalments: the first by 31 January and the second by 31 July. Each instalment is half of your previous year's tax bill, because HMRC assumes you'll earn a similar amount the following year.

If your income changes, you may need to make a balancing payment (if your actual bill is higher) or claim a refund (if it's lower). You can also ask HMRC to reduce your payments on account if you know your income will be lower than the previous year.

How to pay your Self Assessment tax bill

Once you've filed your return and know how much you owe, you'll need to pay by 31 January to avoid late payment penalties. HMRC accepts several payment methods.

The fastest ways to pay are:

• online or telephone banking: same-day or next-day payment using your tax reference number
• CHAPS (Clearing House Automated Payment System): same-day bank transfer
• debit card: pay online through HMRC's payment service
• Direct Debit: set up a single or recurring payment through your HMRC online account
• HMRC app: pay directly from your phone

If you need more time to pay, contact HMRC before the deadline to discuss a Time to Pay arrangement.

Do I need accounting software to file Self Assessment tax returns?

Right now, you can file your annual Self Assessment through HMRC's online gateway or by paper return. But Making Tax Digital (MTD) for Income Tax is changing how self-employed people and landlords report their income.

The rollout happens in three phases based on income level:

• April 2026: sole traders and landlords earning over £50,000 must use MTD-compatible software
• April 2027: the threshold drops to £30,000
• April 2028: the threshold drops to £20,000

Under MTD, the annual Self Assessment return will be replaced by four quarterly digital updates and a final declaration. You'll need software that can connect to HMRC and submit records digitally.

Even if you're not yet required to use software, keeping digital records now can save you time. Cloud accounting software organises your income and expenses as you go, so you're not scrambling at year end to pull everything together.

How long should I keep my Self Assessment records?

HMRC requires you to keep records for at least five years after the 31 January deadline of the relevant tax year. For example, for the 2024/25 tax year (deadline 31 January 2026), you'd need to keep records until at least 31 January 2031.

Records you should keep include:

• bank statements and building society statements
• invoices, receipts, and proof of expenses
• P60 and P45 forms from employers
• records of income from self-employment, property, or investments
• any correspondence with HMRC about your tax return

You can keep these records on paper or digitally. Storing them digitally makes it easier to find what you need if HMRC opens an enquiry into your return.

Getting help with your Self Assessment

If you get stuck or have questions while filing, there are several places to find support.

HMRC offers a range of free resources:

• Self Assessment helpline: call 0300 200 3310 (open Monday to Friday, eight am to six pm)
• HMRC digital assistant: available on the GOV.UK website for quick answers to common questions
• GOV.UK guidance: detailed help pages, webinars, and helpsheets covering every section of the tax return

If you'd prefer one-to-one support, an accountant or bookkeeper can guide you through the process and file on your behalf. This can be a good option if your tax affairs are more complex or you'd rather spend your time running your business.

Simplify your Self Assessment with Xero

Filing a Self Assessment tax return is much easier when your financial records are already organised. Xero keeps your income, expenses, and receipts in one place throughout the year, so you're not left searching for paperwork at the last minute.

With Xero, automated bank feeds pull transactions directly into your account, and you can reconcile them in a few clicks. Expense tracking and receipt capture through Hubdoc mean your records are always up to date and ready when you need them.

Xero is also compatible with Making Tax Digital, so as MTD for Income Tax rolls out, you'll already have the software in place to meet the new requirements. And if you work with an accountant or bookkeeper, they can access your Xero account in real time to prepare and file your return more efficiently.

To see how Xero can help you stay on top of your finances year-round, Get one month free.

FAQs on Self Assessment tax returns

If you're filing for the first time or your income has changed, you may have additional questions. Below are frequently asked questions about Self Assessment tax returns.

What is a Self Assessment tax return for?

Self Assessment is the system HMRC uses to collect income tax that isn't automatically deducted through PAYE. You submit a tax return to report your income and any capital gains for the tax year, so HMRC can calculate how much tax you owe or whether you're due a refund.

Can I do my own Self Assessment tax return online?

Yes. You'll need a Government Gateway account to access HMRC's online service, and you can save your progress and return to it before the deadline.

How much do you need to earn to do a Self Assessment tax return?

If you're self-employed, you must file a tax return if your trading income was more than £1,000 in the tax year. Employees need to file if they earned over £150,000 through PAYE, received untaxed income above certain thresholds, or had other circumstances that require reporting to HMRC.

What happens if I make a mistake on my tax return?

You can correct mistakes online through the Government Gateway up to 12 months after the filing deadline. Genuine mistakes won't usually result in a penalty.

Can I submit my Self Assessment tax return early?

Yes; you can file as soon as the tax year ends on 5 April, though your payment deadline remains 31 January.

What happens if I can't pay my tax bill?

HMRC will assess your income and expenditure before agreeing a repayment plan. Interest continues to accrue during the arrangement, so paying as early as you can reduces the total cost.