The Beginner’s Guide to Self Assessment

What is Self Assessment?

In many cases, HMRC collects tax before the money reaches your bank account. This happens through the Pay As You Earn (PAYE) system. For example, if you earn £3,000 a month from your job, you might only receive £2,400. That’s because £600 in tax and National Insurance has already been taken from your pay and paid directly to HMRC.

PAYE doesn’t apply to all types of income, and you may receive some income untaxed. You’ll need to calculate the tax yourself on this income and report it through the Self Assessment system. For example, you rent out a property for £3,000 a month. PAYE doesn’t apply, so you’ll receive the full £3,000. It’s then your responsibility to report that income and pay any tax due through Self Assessment.

What is a Self Assessment tax return?

A tax return is the form you submit to HMRC to report your income. It helps work out how much tax you owe for a tax year (which runs from 6 April to 5 April the following year).

The tax return has two parts:

• The main return (SA100): This is the standard 8-page form where you can report common income such as employment income
• Supplementary pages: These pages allow you to report additional income such as property income

Do you need to file a tax return?

Most people don’t need to file a tax return because all their income is taxed through the PAYE system. However, if any of the following apply to you, you might have a filing obligation:

• You’re self-employed
• You rent out a property
• You receive dividends or interest
• You’ve made a profit from selling assets such as shares or property
• You’re claiming tax reliefs, for example on pensions or expenses
• You’ve received income from overseas

This is by no means an exhaustive list. You can find more detail on who needs to file here.

Why should you file a tax return?

• If you have been issued a ‘notice to complete a tax return’ from HMRC, you are legally obligated to do so
• HMRC will charge penalties if you don’t file on time
• Submitting a tax return can serve as proof of income when applying for mortgages or business loans
• You might be eligible for tax reliefs and allowances that could result in a tax refund

How do you register with HMRC?

A ‘Notice to complete a tax return’ is HMRC’s formal request for you to file a tax return. If you have received one, you don’t need to notify HMRC further – they will be expecting your tax return by the deadline.

If you haven’t received a notice but believe you do need to file a tax return, you must tell HMRC by 5 October following the end of the tax year. Once you’ve registered, HMRC will send you a formal notice confirming your filing requirement.

For example, if you think you need to file a tax return for the 2025/26 tax year but haven’t filed one before and haven’t been contacted by HMRC, you must inform them by 5 October 2026.

Once you’ve been issued with a notice, HMRC will continue to send one each year. If your circumstances have changed and you no longer need to file a tax return, you can ask HMRC to cancel your filing requirement.

When do you need to file your tax return?

You can file from 6 April following the end of the tax year. This means the earliest you can file your return for the 2025/26 tax year is 6 April 2026.

Paper tax returns must be submitted by 31 October following the end of the tax year. For the 2025/26 tax year, the deadline is 31 October 2026.

Online tax returns are due by 31 January after the end of the tax year. For 2025/26, the deadline is 31 January 2027.

If you’ve received a ‘Notice to complete a tax return’ and miss the deadline, you’ll get an automatic £100 late filing penalty, even if you don’t owe any tax. Further penalties will be applied the longer you delay your submission.

How do you complete your tax return?

You can file your tax return yourself or ask a tax adviser to prepare it for you. Regardless of your choice, it is your responsibility to make sure the return is complete and accurate.

If you’re doing it yourself, HMRC offers a step-by-step guide to help you through the process. You’ll need to provide information on your income, expenses, and any reliefs or allowances you want to claim. Most people file online using HMRC’s system, but you can also submit a paper return if you prefer.

When do you need to pay your tax bill?

You must pay any tax due for the tax year by 31 January following the end of the tax year. For the 2025/26 tax year, you must make payment by 31 January 2027.

If you owe more than £1,000, HMRC may ask you to start making tax payments towards the next year’s bill. These are called payments on account, and it’s HMRC’s way of collecting tax earlier.

Each payment on account is 50% of your previous year’s tax bill, and they’re due in two instalments on 31 January and 31 July.

For example, if your 2025/26 tax bill is £5,000:

• On 31 January 2027, you pay the £5,000 due for 2025/26 plus your first payment on account of £2,500 towards the 2026/27 tax bill
• On 31 July 2027, you pay your second payment on account of £2,500

When you complete your 2026/27 tax return, you subtract the payments on account from the total tax due. So if your 2026/27 bill comes to £7,000, you’ll deduct the two £2,500 payments already made. This leaves a balancing payment of £2,000 which is due by 31 January 2028.

If you’re late paying your tax, HMRC will charge penalties based on how much is overdue and how long it has remained unpaid. They’ll also charge interest on any late payments of tax, including payments on account.