Self Assessment Tax Return

Self Assessment is the system HMRC uses to collect Income Tax and National Insurance from individuals and businesses whose tax is not fully deducted at source through PAYE. Each year, millions of people in the UK complete a Self Assessment tax return — known as the SA100 form — to report their income and gains. The UK tax year runs from 6 April to 5 April, and the return for the previous year is due by specific deadlines.

Who Must File a Self Assessment Tax Return

You are required to file a Self Assessment return if you are self-employed as a sole trader and your gross turnover exceeds £1,000, you are a partner in a business partnership, you have untaxed income over £2,500 (such as tips, commission, rental income, or investment income where tax was not deducted at source), your total income exceeds £100,000 (which triggers a reduction in your Personal Allowance), you receive Child Benefit and your adjusted net income (or your partner's) is over £50,000, you are a company director, or you have made a Capital Gain on an asset that requires reporting. You may also need to file if HMRC sends you a notice to file, even if you do not believe you owe any tax. If you are unsure, use the HMRC online tool "Check if you need to send a tax return" on GOV.UK.

Registering for Self Assessment

If you have never filed a Self Assessment return before, you must register with HMRC by 5 October following the end of the tax year. For the 2025/26 tax year, that means registering by 5 October 2026. You can register online at GOV.UK. You will need your National Insurance number and details about your income. After registering, HMRC posts you a letter containing your Unique Taxpayer Reference (UTR) — a 10-digit number that you use every time you file. You then activate your Government Gateway account to file online. Registration is free.

Deadlines: 31 October (Paper) and 31 January (Online)

There are two key filing deadlines each year. If you file a paper tax return, it must reach HMRC by 31 October following the end of the tax year. For 2025/26, that is 31 October 2026. Paper returns use the SA100 form and supplementary pages (such as SA103 for self-employment, SA105 for property income). If you file online, you have until 31 January following the end of the tax year. For 2025/26, that is 31 January 2027. Online filing is recommended: the HMRC software calculates your tax automatically, you can save and resume your return, and you have three extra months compared with paper filing.

Filing Online vs Paper

Online filing through HMRC's Government Gateway portal is the most popular method. You can also file through third-party commercial software that integrates with HMRC's systems. Online filing pre-populates some information (such as employment income from your P60 data) and performs real-time validation checks. Paper filing involves completing the SA100 form and any supplementary pages by hand or typewriter, then posting them to HMRC. Paper returns are more error-prone because you must manually calculate your tax liabilities and allowances. Since April 2026, Making Tax Digital for Income Tax (MTD IT) is mandatory for sole traders and landlords whose gross income exceeds £50,000. For anyone else, the choice remains open.

What Information You Need

Before you start your return, gather your P60 (employment income summary), P11D (benefits and expenses), P45 (if you left a job), bank statements showing interest earned, dividend vouchers for any shares you own, records of self-employed income and expenses, details of rental income and property expenses, capital gains records for any assets sold, pension contribution statements, and gift aid donation receipts. Keep all records for at least five years after the filing deadline — HMRC can request them at any time.

Paying Your Tax Bill

Any tax you owe for the 2025/26 tax year is due by 31 January 2027. You can pay by online bank transfer (Faster Payments), debit card, direct debit, CHAPS, or at your bank using the payslip from HMRC. If you cannot pay in full, you can apply for a Time to Pay arrangement online through your HMRC account for debts up to £30,000. Interest is charged on late payments at the Bank of England base rate plus 2.5%.

Penalties for Late Filing and Late Payment

HMRC imposes automatic penalties for late filing. If your return is late by one day, you incur a £100 fixed penalty. If it is still late after three months, additional penalties of £10 per day apply for up to 90 days (maximum £900). After six months, a further penalty of 5% of the tax due or £300, whichever is greater. After twelve months, another 5% or £300, and in serious cases HMRC can charge up to 100% of the tax due. Late payment penalties are also charged in tiers: 5% of the unpaid tax after 30 days, another 5% after six months, and another 5% after twelve months — a maximum of 15% in late payment penalties, plus daily interest. If you have a reasonable excuse (serious illness, bereavement, technical failure), you can appeal against penalties.

Need help navigating UK taxes? Browse all UK guides or try our calculators.

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