Self Assessment: the complete guide
Who must file, registration and deadlines, payments on account, the supplementary pages, penalties and the 2025-26 allowances.
Self Assessment is how HMRC collects tax on income that isn't already taxed through your wages. If you're self-employed, a landlord, a higher earner or an investor, the odds are you need to file one. This guide covers who has to, the deadlines, how payments on account work, and which extra pages you might need.
Do you have to file?
You generally need to send a Self Assessment return for 2025-26 if any of these apply:
- You were self-employed with gross trading income over £1,000 (the trading allowance).
- You were a partner in a business partnership.
- You had to pay the High Income Child Benefit Charge and don't pay it through PAYE.
- You had untaxed income — rent, tips, commission, or savings/dividend income above the allowances.
- You made capital gains that need reporting.
- You had foreign income, or HMRC sent you a notice to file.
Registering and getting a UTR
If you're new to Self Assessment, you must tell HMRC by 5 October following the tax year you first need to file — so for 2025-26, by 5 October 2026. HMRC then issues your Unique Taxpayer Reference (UTR), a 10-digit number you'll use on every return, and enrols you for online filing. Leave time for the codes to arrive by post before the deadline.
The deadlines that matter
| What | Deadline |
|---|---|
| Register for Self Assessment | 5 October 2026 |
| Paper return | 31 October 2026 |
| Online return | 31 January 2027 |
| Pay the tax you owe | 31 January 2027 |
| First payment on account (next year) | 31 January 2027 |
| Second payment on account | 31 July 2027 |
Payments on account
This is the part that surprises first-time filers. If your last bill was over £1,000 and less than 80% of your tax was collected at source, HMRC asks you to pay toward next year's bill in advance, in two instalments each equal to half your previous year's liability — due 31 January and 31 July. A balancing payment then settles the difference the following January.
The supplementary pages
The SA100 is the core return. You add “supplements” for specific income types:
| Form | When you need it |
|---|---|
| SA102 | Employment income, benefits and expenses |
| SA103S / SA103F | Self-employment (short / full) |
| SA105 | UK property and rental income |
| SA106 | Foreign income and foreign tax credit relief |
| SA108 | Capital gains |
| SA109 | Residence and domicile |
| SA101 | Additional information — EIS/VCT, life-insurance gains, certain reliefs |
Allowances for 2025-26
The Personal Savings Allowance is £1,000 for basic-rate taxpayers, £500 for higher-rate and nil for additional-rate. Many thresholds are frozen, so more people are dragged into filing each year — the definition of fiscal drag.
Penalties for filing or paying late
- £100 the moment you miss the filing deadline — even if you owe no tax.
- £10 a day after three months (up to £900).
- At 6 and 12 months, a further £300 or 5% of the tax, whichever is higher, each time.
- Late payment: 5% of the unpaid tax at 30 days, again at 6 months and at 12 months, plus daily interest (currently base rate + 4%).
How Making Tax Digital changes things
From April 2026, sole traders and landlords above the qualifying-income thresholds move off the annual SA100 and onto MTD for Income Tax — digital records, quarterly updates and a Final Declaration. The 31 January payment dates and payments on account stay exactly the same. Everyone below the thresholds, and partnerships for now, continue with the standard Self Assessment process described here.
Sources & further reading
This guide is general information, not personal tax advice, and reflects the rules we believe to apply as at June 2026 — rates and thresholds change. Always check your own figures against HMRC and consider a qualified adviser before acting. You remain responsible for the accuracy of anything you file.
File Self Assessment without the dread
Pull your figures from HMRC, your calculators and your bank, validate the numbers, and file your SA100 — all in one place.
Start freeFrequently asked questions
Who has to file a Self Assessment tax return?
You generally must file if you were self-employed earning over £1,000, a partner in a partnership, a company director with untaxed income, or had untaxed income from property, savings, dividends or abroad — or if you earned over £150,000 or owe the High Income Child Benefit Charge. HMRC’s online tool confirms your position.
What is the Self Assessment deadline?
Register by 5 October after the tax year ends; file online and pay any tax due by 31 January. Paper returns are due earlier, by 31 October. So the 2024-25 return must be filed online by 31 January 2026.
What is the penalty for filing Self Assessment late?
An automatic £100 penalty applies the day after the deadline even if you owe nothing, then £10 daily penalties after three months, with further penalties at six and twelve months — plus interest on any tax paid late.
What are payments on account?
Advance payments toward next year’s bill, each 50% of this year’s tax, due 31 January and 31 July. You don’t make them if your last bill was under £1,000 or if more than 80% of your tax is collected at source.
Can I file my Self Assessment myself?
Yes — through HMRC’s online service or recognised software. AI TaxPilot pre-fills the SA100 from HMRC and your bank, checks the figures, and submits the return for you.