How to Complete a Self Assessment Tax Return

Step 1: Register and get your UTR

If this is your first return, register for Self Assessment by 5 October after the end of the tax year. For 2026/27 that means by 5 October 2027. Self-employed people register as a sole trader, which also sets up Class 2 and Class 4 National Insurance.

HMRC then posts your 10-digit Unique Taxpayer Reference (UTR). You'll also create a Government Gateway user ID. If you are new to the online service, HMRC posts an activation code too, so leave yourself time before the deadline.

The Self Assessment timeline: every date from registration to payment

Step 2: Gather what you need

Having everything to hand before you log in saves a lot of back and forth.

• Your UTR and National Insurance number
• P60 or P45 from any job, and P11D if you had company benefits
• Self-employed income and allowable expenses for the year (6 April to 5 April)
• Rental income and costs if you're a landlord
• Bank and building society interest, and dividend statements
• Details of any assets sold, for Capital Gains Tax
• Pension contributions you paid personally and Gift Aid donations
• Student loan plan type, if you have one

Step 3: Log in and choose your sections

Sign in through GOV.UK with your Government Gateway details or the HMRC app. The online return starts with a set of questions about your year. Your answers decide which sections appear, such as employment, self-employment, property or capital gains. Answer carefully, because a missed 'yes' hides a section you need.

Step 4: Enter your income

Enter each source in its section. Employment pay and tax taken off come from your P60. For self-employment you enter turnover and allowable expenses, and if turnover is under the VAT threshold you can give one total for expenses. If your trading or property income is £1,000 or less, the trading or property allowance may mean you don't need to report it at all.

Use the Self Employed Tax Calculator to see what you would save

Step 5: Claim reliefs and allowances

Higher and additional rate taxpayers can claim extra relief on personal pension contributions and Gift Aid here. Basic rate relief is usually already added by the pension provider or charity, so you only enter the gross amount once. You can also claim Marriage Allowance and work expenses if they apply.

Step 6: Check the calculation and submit

Before submitting, the online service shows a tax calculation. Compare it with your own estimate and look for typos in large figures. Once you submit, save the confirmation and submission reference. You can amend an online return for up to 12 months after the 31 January deadline.

Use the Self Assessment Calculator to see what you would save

Step 7: Pay what you owe

The balancing payment is due by 31 January. You may also have to make payments on account towards next year: two instalments, each half of this year's bill, due 31 January and 31 July. These aren't needed if your bill is under £1,000 or more than 80% of your tax was already taken at source, such as through PAYE.

Common mistakes to avoid

• Leaving registration too late for the activation code to arrive
• Entering monthly instead of yearly figures
• Forgetting interest, dividends or a second job
• Claiming basic rate pension relief twice
• Missing the 31 January payment even when the return is filed on time