Self Assessment is HMRC's system for collecting tax on income that has not been taxed at source. If you are self-employed, a landlord, a company director, or have income above certain thresholds, you are likely required to register and file a return each year. For the 2025/26 tax year, the online filing deadline is 31 January 2027 and the payment deadline is the same date.
What is Self Assessment?
Self Assessment is HMRC's framework for declaring income and calculating the tax owed on it. Unlike employees taxed through PAYE, Self Assessment taxpayers are responsible for reporting their own income, calculating their own tax liability, and paying any amount due. You do this by completing a Self Assessment tax return, either online through HMRC's Government Gateway or on a paper SA100 form.
For most people, the online return is the better option — it calculates your tax liability automatically, has a later filing deadline, and allows you to see your tax bill immediately. Paper returns have a significantly earlier deadline and must be submitted to HMRC by post.
Who must file a Self Assessment return?
HMRC requires you to file a Self Assessment return if any of the following applied in the relevant tax year:
- You were self-employed as a sole trader with gross income over £1,000
- You are a partner in a business partnership
- You received rental income from property (including letting a room above the £7,500 rent-a-room threshold)
- Your total taxable income exceeded £100,000
- You received dividends above the £500 dividend allowance
- You received untaxed income over £2,500 (such as tips or commission not put through payroll)
- You or your partner claimed Child Benefit and either of you earned over £60,000 (the High Income Child Benefit Charge threshold)
- You have income from abroad that is taxable in the UK
- You are a company director who received income not taxed through PAYE
- You need to claim certain reliefs, including higher-rate Gift Aid tax relief or pension contribution relief above the basic rate
- HMRC has sent you a notice to file — once issued, you must file the return even if you have no tax to pay
Read the full guide: Who needs to file a Self Assessment tax return?
How to register for Self Assessment
If you have never filed before, you must register with HMRC before you can submit a return. The method varies depending on your circumstances:
- Self-employed sole trader: Register online at GOV.UK using form SA1 or through the Government Gateway. HMRC will set up your Self Assessment record and issue a Unique Taxpayer Reference (UTR) number within 10 working days.
- Partnership: Each partner registers individually for Self Assessment, and a nominated partner registers the partnership itself using form SA400.
- Company director or landlord: Register using form SA1 on GOV.UK.
Your UTR is a 10-digit reference number unique to you. Keep it safe — you need it every time you file a return, contact HMRC about your tax, or authorise an agent to act on your behalf.
You must register by 5 October following the end of the tax year in which you first needed to file. For the 2025/26 tax year, the registration deadline is 5 October 2026.
Key deadlines for the 2025/26 tax year
| Deadline | Date | What it applies to |
|---|---|---|
| Register for Self Assessment | 5 October 2026 | If you need to file for 2025/26 and have not filed before |
| Paper return filing | 31 October 2026 | SA100 paper return for 2025/26 |
| Online return filing | 31 January 2027 | Self Assessment online return for 2025/26 |
| Tax payment | 31 January 2027 | All tax owed for 2025/26 plus first payment on account for 2026/27 |
| Second payment on account | 31 July 2027 | Second payment on account for 2026/27 |
What to include in your Self Assessment return
The SA100 is the main Self Assessment return. Depending on your circumstances, you may also need to complete supplementary pages:
- SA103S/SA103F — Self-employment income (short or full version)
- SA104 — Partnership income
- SA105 — UK property income
- SA106 — Foreign income
- SA108 — Capital Gains
- SA109 — Residence and remittance basis
You must report all income received in the tax year, including employment income (even if already taxed through PAYE), savings interest, dividends, rental income, and business profits. You can also claim allowable deductions and reliefs, including business expenses, pension contributions, charitable donations under Gift Aid, and professional subscriptions.
Payments on account
If your Self Assessment tax bill is over £1,000 and less than 80% of your total tax was collected at source through PAYE, HMRC requires you to make advance payments towards next year's tax. These are called payments on account.
Each payment on account is 50% of your previous year's tax bill, paid in January and July. When you file your return, any over- or underpayment is reconciled.
For example, if your 2025/26 tax bill is £6,000, you pay:
- 31 January 2027: £6,000 (2025/26 tax) + £3,000 (first payment on account for 2026/27) = £9,000
- 31 July 2027: £3,000 (second payment on account for 2026/27)
If you expect your 2026/27 income to be lower than 2025/26, you can apply to reduce your payments on account. Be careful — if you underestimate and pay too little, HMRC will charge interest on the shortfall.
Penalties for late filing and late payment
HMRC's penalty regime is strict and automatic. Penalties begin the moment you miss a deadline, even if you owe no tax.
| How late | Penalty |
|---|---|
| 1 day late (filing) | £100 automatic penalty |
| 3 months late (filing) | £10 per day for up to 90 days (maximum £900) |
| 6 months late (filing) | 5% of tax due, or £300 — whichever is greater |
| 12 months late (filing) | Further 5% of tax due, or £300 — whichever is greater |
| 30 days late (payment) | 5% of tax unpaid |
| 6 months late (payment) | Further 5% of tax unpaid |
| 12 months late (payment) | Further 5% of tax unpaid |
Interest also accrues daily on unpaid tax from the date it was due. If you cannot pay, contact HMRC as early as possible to arrange a Time to Pay agreement before penalties escalate.
Practical tips to make Self Assessment filing easier
- Keep records throughout the year. Store receipts, invoices, bank statements, and allowable expense records as you go. Trying to reconstruct 12 months of records in January is unnecessarily stressful.
- File early. Filing in April or May gives you the tax figure months before the January payment date, allowing you to plan your cash flow.
- Check your tax code. If you are also employed, verify that your PAYE tax code is correct. Errors in your tax code can mean you have underpaid or overpaid throughout the year, affecting your Self Assessment bill.
- Claim all allowable expenses. Self-employed people can deduct costs wholly and exclusively incurred for business. Common deductions include mileage (45p per mile for the first 10,000 miles), home office costs, professional subscriptions, and accountancy fees.
- Consider using an accountant. For complex returns — multiple income sources, capital gains, overseas income — a qualified accountant typically saves more in tax than they cost in fees.
Key takeaways
- You must file a Self Assessment return if you are self-employed (over £1,000 gross), a partner, a landlord, a director, or earn over £100,000 — among other triggers.
- Register for Self Assessment by 5 October 2026 if 2025/26 is your first year; your UTR number will arrive within 10 working days.
- The online filing deadline for 2025/26 is 31 January 2027; the paper deadline is 31 October 2026.
- If your tax bill exceeds £1,000, you must make payments on account in January and July towards the following year's liability.
- Penalties start the day after a missed deadline — £100 automatically, rising to daily charges after three months.
Frequently asked questions
Do I need to file a Self Assessment return if I have no tax to pay?
Possibly, yes. If HMRC has sent you a notice to file, you must submit a return regardless of whether you have tax to pay. Failing to do so results in automatic penalties starting at £100. You can ask HMRC to close your Self Assessment record if your circumstances have changed and you no longer need to file, but this must be agreed before the deadline.
What is a UTR number and where do I find it?
A UTR (Unique Taxpayer Reference) is a 10-digit number that identifies you in HMRC's Self Assessment system. It appears on any correspondence from HMRC about Self Assessment, on previous tax returns, and in your HMRC online account. If you have lost your UTR, you can retrieve it by calling HMRC's Self Assessment helpline or logging into your Personal Tax Account.
Can I correct a Self Assessment return after I have filed it?
Yes. You can amend a Self Assessment return within 12 months of the original filing deadline. For a 2025/26 return filed in January 2027, you have until 31 January 2028 to make corrections. After this window closes, you must contact HMRC directly to request an amendment, which is assessed on a case-by-case basis.
What expenses can I deduct as a self-employed person?
You can deduct costs that are wholly and exclusively incurred for your business. Common deductible expenses include business mileage (45p per mile for the first 10,000 miles, then 25p), home office costs (using simplified expenses or actual costs), professional subscriptions, accountancy fees, business insurance, advertising, equipment, and training directly related to your current trade.
What happens if I cannot pay my Self Assessment bill?
Contact HMRC as soon as possible and do not wait until the deadline passes. HMRC may agree a Time to Pay arrangement, which spreads the payment over monthly instalments. Interest will still accrue on the outstanding balance, but the penalty regime for late payment can be avoided if you set up an arrangement before the penalty trigger dates.