Self-Assessment is HMRC's annual mechanism for collecting income tax and National Insurance on income that has not already been taxed at source. Twelve million UK taxpayers file every year: sole traders, landlords, high earners, partnership members, company directors with non-PAYE income, and anyone with foreign income. The system has been in place since 1996-97 but the mechanics of registration, filing, payments on account, and the SA302 mortgage trap continue to catch out new filers.
This guide walks through the entire Self-Assessment process end to end. Each section links to a detailed companion piece on the specific issue.
The 31 January deadline is firm
Online Self-Assessment returns and any tax payment due are filed and paid by 31 January following the end of the tax year. £100 is automatic on day one. After three months, £10 per day adds up to £900. Then 5% percentage penalties at six and twelve months. There is no soft-edge to the deadline.
Who needs to file Self-Assessment
Self-Assessment applies to:
- Sole traders with gross income above £1,000 after the trading allowance.
- Landlords with rental income above £1,000 after the property allowance.
- Partners in a partnership.
- Company directors receiving dividends or other income beyond fully PAYE-taxed salary.
- High earners above £150,000 in some circumstances.
- Anyone with capital gains above the £3,000 annual exempt amount.
- Anyone with foreign income.
- Anyone with adjusted net income above £60,000 receiving Child Benefit (HICBC).
- Anyone who has received a notice from HMRC requiring a return.
Registering for Self-Assessment and the UTR
New filers must register with HMRC before the deadline of 5 October following the end of the relevant tax year. The process:
- 1Create a Government Gateway account at gov.uk/log-in-register-hmrc-online-services.
- 2Apply for Self-Assessment registration. Sole traders use form CWF1; non-self-employed filers use form SA1.
- 3HMRC issues a 10-digit Unique Taxpayer Reference (UTR) within 10 working days, posted to the registered address.
- 4A separate activation code arrives by post for the online filing service.
- 5Once activated, the filer can submit returns from the personal tax account.
The UTR is the lifelong tax identifier
Once issued, your UTR stays with you for life. Even if you stop filing Self-Assessment for a few years and resume later, the same UTR applies. Lost UTRs can be retrieved via the personal tax account or HMRC helpline.
Paper vs online filing
Two filing routes:
Self-Assessment filing: paper vs online
| Aspect | Paper return | Online return |
|---|---|---|
| Deadline | 31 October | 31 January |
| Calculation | Manual or via accountant | HMRC auto-calculates |
| Receipt confirmation | Posted notice, slow | Instant on-screen |
| Amendment window | 12 months | 12 months via online amendment |
| MTD ITSA from April 2026 | Not available | Required (with software) |
Paper filing is being phased out. From April 2026 most landlords and sole traders above the MTD threshold will need to file digitally. For those still using paper, the 31 October deadline gives three extra months of preparation time but the eligibility window is narrowing each year.
The SA302 and why mortgage lenders need it
The SA302 is HMRC's official tax calculation summary for the year. Mortgage lenders working with self-employed applicants typically request SA302s for the last 2-3 years to verify income. Where to access:
- Online via the HMRC personal tax account: navigate to Self-Assessment → previous years → view tax calculation.
- Via your accountant if they file on your behalf.
- For paper-filed returns, HMRC posts SA302s on request (slower).
Lenders also typically require a Tax Year Overview alongside the SA302. The Overview confirms the tax has actually been paid, while the SA302 shows the calculation. Without both, mortgage applications stall at the income verification stage.
Correcting an error on a submitted return
Errors can be corrected without penalty if disclosed promptly:
- 1Within 12 months of the original filing deadline: amend the return online via the personal tax account.
- 2Beyond 12 months but within 4 tax years: write to HMRC requesting an "overpayment relief" claim.
- 3Beyond 4 tax years: only HMRC discovery assessment process can adjust the return, and only in defined circumstances.
- 4Where additional tax is owed, interest applies from the original payment due date.
- 5Where tax was overpaid, HMRC repays with interest from the original payment date.
The Self-Assessment Series
We're publishing two detailed pieces per week from this series. Check back shortly.
Tax-free allowances 2026-27
The major allowances on the 2026-27 return:
- Personal allowance: £12,570 (frozen until April 2028).
- Trading allowance: £1,000 of gross self-employment income tax-free.
- Property allowance: £1,000 of gross rental income tax-free.
- Dividend allowance: £500 (reduced from £1,000 in April 2024).
- Personal Savings Allowance: £1,000 (basic rate), £500 (higher rate), £0 (additional rate).
- Capital Gains Tax annual exempt amount: £3,000 (down from £12,300 pre-2023).
- Marriage Allowance: up to £252 transfer between spouses where one is a non-taxpayer.
The penalty escalator: missed 31 January
Penalties for late filing escalate fast:
Self-Assessment late filing penalties
| Time after deadline | Late filing penalty |
|---|---|
| 1 day | £100 fixed |
| 3 months | Plus £10 per day, max £900 |
| 6 months | Plus 5% of tax due or £300 (greater) |
| 12 months | Plus a further 5% of tax due or £300 (greater) |
Late payment interest accrues at the HMRC prevailing rate (currently 7.75% in 2026) from 1 February. A 5% surcharge applies to unpaid tax at 28 days, with further surcharges at 6 and 12 months. Late filing penalties stack with late payment penalties: a six-month late filer with unpaid tax can find themselves £400+ in penalties before any interest is calculated.
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