What is Self Assessment?
Self Assessment is the system HMRC uses to collect Income Tax from people whose tax is not automatically deducted from wages or pensions. If you are self-employed, a landlord, or have other untaxed income, you report it on a Self Assessment tax return (the SA100) once a year and pay any tax due.
In short: PAYE handles tax for most employees automatically. Self Assessment is how everyone else tells HMRC what they earned and what they owe.
Who needs to file a Self Assessment return?
You must usually file a Self Assessment return for the 2025/26 tax year if any of the following applied:
- You were self-employed as a sole trader and earned more than £1,000 (before expenses) — the trading allowance
- You were a partner in a business partnership
- You received rental income above the £1,000 property allowance
- You had to pay the High Income Child Benefit Charge
- You received untaxed income — tips, commission, savings, investments or dividends above your allowances
- You earned over £150,000 through PAYE
- You made capital gains above the annual exempt amount
- You had foreign income taxable in the UK
Construction subcontractors taxed under CIS almost always file Self Assessment to reclaim overpaid deductions — see our CIS deductions guide and free CIS calculator.
Key Self Assessment deadlines
Miss these and HMRC charges automatic penalties, so put them in your calendar now.
| Deadline | What is due |
|---|---|
| 5 October 2026 | Register for Self Assessment (first-time filers) |
| 31 October 2026 | Paper tax return |
| 31 January 2027 | Online return and payment of tax owed for 2025/26 |
| 31 January 2027 | First payment on account for 2026/27 |
| 31 July 2027 | Second payment on account for 2026/27 |
Most people file online by 31 January. See the full year in our UK tax deadline calendar.
How to file your Self Assessment online — step by step
Step 1 — Register and get your UTR If it's your first return, register with HMRC for Self Assessment. They post you a Unique Taxpayer Reference (UTR) within about 10 working days, then an activation code for your Government Gateway account. Do this well before the 5 October deadline.
Step 2 — Gather your figures You need your total income and allowable expenses for the tax year (6 April to 5 April), plus any other income — bank interest, dividends, P60/P45 from employment, pension contributions and Gift Aid donations.
Step 3 — Complete the return Log in to your HMRC account, open the SA100 and the relevant supplementary pages (SA103 for self-employment, SA105 for property). Enter income and expenses by category. HMRC calculates the tax owed automatically as you go.
Step 4 — Check and submit Review the calculation, check for obvious errors (a misplaced decimal is the classic mistake), then submit. You get an on-screen confirmation and reference number — keep it.
Step 5 — Pay Pay what you owe by 31 January, by bank transfer, debit card or Direct Debit. If your bill is large, you may also owe a payment on account.
Allowable expenses — what you can claim
You only pay tax on your profit, so claiming legitimate business expenses reduces your bill. Common allowable expenses for sole traders:
- Office costs, stationery, phone and internet
- Travel and mileage (45p per mile for the first 10,000 business miles)
- Stock and raw materials
- A proportion of home costs if you work from home
- Professional fees, insurance and bank charges
- Marketing and software subscriptions
Keep a digital record and a copy of every receipt — HMRC can ask you to evidence any expense for up to six years. Our expense management guide covers how to stay on top of this.
Payments on account explained
If your tax bill is more than £1,000 and less than 80% of your tax is collected at source, HMRC asks you to make payments on account — two advance payments toward next year's bill, each 50% of this year's tax, due 31 January and 31 July.
This often catches first-time filers out: in your first January you can effectively pay 150% of your bill (the year you owe plus the first payment on account). Budget for it.
How Making Tax Digital changes Self Assessment
From April 2026, Making Tax Digital for Income Tax Self Assessment (MTD ITSA) begins replacing the annual return for sole traders and landlords with quarterly digital updates. It applies to gross income above £50,000 from April 2026, above £30,000 from April 2027, and above £20,000 from April 2028.
If that is you, the once-a-year SA100 becomes four quarterly submissions plus a year-end finalisation, all through MTD-compatible software. Use our free MTD checker to confirm your start date, and read the full Making Tax Digital guide for what changes.
How Finovo helps
Finovo keeps your income and expenses categorised throughout the year from your connected bank feed, so your Self Assessment figures are ready when you need them — no shoebox of receipts in January. When MTD ITSA applies to you, Finovo files the quarterly updates directly to HMRC.
Start a free 60-day trial and connect your bank account in minutes.