How to file a self assessment tax return online: step by step

In short

• How to file a self assessment tax return: you file it online through your HMRC account on GOV.UK. You need your sign-in details, your UTR and documents showing your income, such as your P60.
• First time: you register for Self Assessment first. If the registration deadline has already passed, HMRC will tell you your deadline in a letter or email.
• Your UTR has 10 digits: after registering online it appears in your personal tax account within 72 hours, and after registering by post it usually arrives within 15 working days (as of November 2026, GOV.UK).
• Payment and payments on account: your payment reference is your UTR followed by the letter "K". You pay payments on account on 31 January and 31 July.

What to have ready before you start

The 2025/26 return covers the period from 6 April 2025 to 5 April 2026. The usual online filing deadline for 2025/26 is 31 January 2027; our income tax guide covers registration, paper returns and exceptions (as of November 2026, GOV.UK).

Have ready:

• your HMRC sign-in details and your UTR (both explained below),
• photo ID (a passport or driving licence): HMRC may ask you to prove your identity,
• records of your income and costs, such as bank statements and receipts,
• your P60, P45 and P11D, if you have them.

Do you work in construction under CIS? See our guide to CIS in construction: tax, registration and refunds.

How to create an HMRC account and sign in

To sign in to HMRC online services (as of November 2026, GOV.UK), you use either Government Gateway (a user ID of up to 12 characters and a password) or GOV.UK One Login (an email address and password). If you already use HMRC services, sign in with your existing details. If you have never used them, choose "Create new sign in details" when you sign in, even if you have a One Login from another government service.

The HMRC app (GOV.UK) works on iOS and Android. It shows your UTR and the amount of tax you owe, and lets you pay. Watch out for fake messages pretending to be from HMRC: we describe them in our guide to scams in the UK.

How to register for Self Assessment

If you need to file a return and have never done so before, you tell HMRC by registering for Self Assessment (as of November 2026, GOV.UK). The "Check how to register for Self Assessment" tool takes you down one of these routes: self-employed, a partner in a business partnership, or another reason, such as income from renting out a property.

You can save your online registration and finish it later, but you cannot register online if you do not live in the UK. To register by post, use the form:

• CWF1: if you are self-employed (GOV.UK),
• SA1: if you are not self-employed; you give the reason, such as Capital Gains Tax or income from a trust (GOV.UK),
• SA401: if you are a partner in a business partnership; the "nominated partner" also registers the partnership itself (GOV.UK).

Reactivating your account. If you registered before but did not send a return last year, you may need to reactivate your account. A return filed without reactivating may be delayed.

Setting up a business? See our guide to being self-employed in the UK as a sole trader.

Announced change. Budget 2026 announces an increase in the Trading Allowance from £1,000 to £3,000, and the threshold for reporting trading income in Self Assessment is also due to rise to £3,000. The changes are due to take effect from 6 April 2027 and need legislation (Finance Bill 2026-27), so for now this is only an announcement (as of November 2026, GOV.UK).

Where to find your UTR

Your UTR (Unique Taxpayer Reference) has 10 digits and may just be called "tax reference" (as of November 2026, GOV.UK). You can find it in your personal tax account, in the HMRC app, or on earlier tax returns and letters from HMRC, for example notices to file a return. If it is not in any of these places, contact HMRC.

After you register online, your UTR appears in your account within 72 hours. After you register by post, it usually arrives in 15 working days (21 days if you are abroad), and may take longer during busy periods (GOV.UK).

How to file a self assessment tax return online, step by step

You file the return in the "File your Self Assessment tax return online" service (as of November 2026, GOV.UK).

1. Check your registration. Register if you are filing for the first time. If you registered before but did not file last year, use the registration tool to check whether you need to reactivate your account.
2. Sign in with your HMRC details. You also need your UTR. If the service asks, prove your identity.
3. Complete the sections in turn. Each one has help text. You can save your entries and come back later.
4. Add supplementary pages if you have the types of income in the table below.
5. Check your tax calculation (also known as an SA302). If you file online, you can see it in your account before you submit. For up to 72 hours after you submit your return you cannot view it, so if a payment is due in that time, save a copy before you submit.
6. Submit the return before the deadline.

The service will not accept the return of, among others, someone who lived abroad as a non-resident. In that case you use commercial software or forms.

Supplementary pages

The main form is the SA100. For some types of income you must fill in extra sections (GOV.UK):

P60, P45 and P11D

Documents from your employer (as of November 2026, GOV.UK):

• P45: you get one when you stop working for an employer. If you lose your P45, you cannot get a replacement.
• P60: shows the tax you paid on your salary in the tax year, with a separate one for each job. Your employer must give you one by 31 May if you are working for them on 5 April.
• P11D: covers benefits in kind, for example a company car. You can ask your employer for a record of what they reported, including how much each benefit is worth.

You can see pay and tax details for the last 5 tax years in your personal tax account or the HMRC app.

How to correct your return

You can correct a return within 12 months of the Self Assessment deadline, online or by sending another paper return (as of November 2026, GOV.UK). Online, you can only do so 3 days (72 hours) after filing: sign in, then in "Your tax account" choose "Self Assessment account" → "More Self Assessment details" → "At a glance" in the left-hand menu → "Tax return options", then pick the year, make the corrections and file it again. If you miss the deadline or the change is for an earlier tax year, you write to HMRC.

How to pay your tax bill to HMRC

You pay the tax for 2025/26 by 31 January 2027, 11:59pm (as of November 2026, GOV.UK). Paying late means interest and a possible penalty. Your payment reference has 11 characters: your 10-digit UTR followed by the letter "K". With the wrong reference, your payment may be delayed or used to pay a different tax bill.

You cannot pay by personal credit card or at the Post Office. If the deadline falls on a weekend or bank holiday, your payment (except Faster Payments and cards) must reach HMRC on the last working day before. Your bill tells you which account to pay into; without a bill, you can pay into either: sort code 08 32 10, account number 12001039 (HMRC Cumbernauld) or 12001020 (HMRC Shipley).

Planning payments and difficulty paying:

• Budget Payment Plan: weekly or monthly Direct Debit payments towards your next bill. You must be up to date with payments from your last Self Assessment bill to set one up.
• Time to Pay: if you cannot pay the full amount, you may be able to set up a payment plan with HMRC.
• Tax code: eligibility depends on HMRC's conditions and restrictions. Your bill must be less than £3,000; you cannot make a part payment to meet this threshold. Check the full conditions on GOV.UK (as of November 2026).

Payments on account

Payments on account are payments towards your next tax bill, including Class 4 National Insurance if you are self-employed (as of November 2026, GOV.UK). There are two, each usually half of the tax you owed last year, due by midnight on 31 January and 31 July. You do not make them if last year's tax bill was less than £1,000, or if you paid more than 80% of last year's tax outside Self Assessment, for example through your tax code.

If you did not make payments on account last year (for example because it is your first time filing), you pay two amounts at once the first time: the balance for the previous year and your first payment on account. If you know your tax will be lower than last year, you can ask HMRC to reduce your payments on account: online with "Reduce payments on account", or by post with form SA303. If you reduce your payments on account and your bill is higher than expected, HMRC will charge interest on the difference.

You can work out tax and National Insurance on a salary with our take-home pay calculator (2026/27 tax year).

Frequently asked questions

I do not have a UTR. Can I file my return online?

No, you need a UTR to file online. If you have never registered for Self Assessment, start by registering. If you have registered, look for the number in your account, in the app or in old letters from HMRC.

Can someone else fill in my return for me?

Yes. You can appoint someone to fill in and send your tax return, for example an accountant, friend or relative. Voluntary or community sector organisations also offer free Self Assessment help and advice.

What is the penalty for filing late?

The initial penalty for a late return is £100 (as of November 2026, GOV.UK). We cover further penalties and how to appeal in our guide to the HMRC penalty for late Self Assessment.

Where to get help

• HMRC Self Assessment helpline: 0300 200 3310 (from outside the UK +44 161 931 9070), Monday to Friday, 8am to 6pm, closed on bank holidays (as of November 2026, GOV.UK).
• Online: HMRC's digital assistant and a webchat with an adviser, if one is available. HMRC's online services helpdesk deals with technical problems with your account.
• Post: Self Assessment, HM Revenue and Customs, BX9 1AS, United Kingdom.
• Help online with Self Assessment: online help and free help from community organisations.

This guide is general information and not tax advice. Check amounts, deadlines, account numbers and phone numbers on GOV.UK.