Key takeaways
- Most people file online through their HMRC account or commercial software; the deadline for 2025/26 is 31 January 2027.
- Paper returns (SA100) had an earlier deadline of 31 October 2026.
- Gather your P60, P11D, interest and dividend figures, business or rental records, and pension and Gift Aid payments before you start.
- Sole traders and landlords in Making Tax Digital file their return through MTD-compatible software instead.
- Check HMRC's calculation, then pay by 31 January, including any first payment on account.
Filing a Self Assessment return is straightforward once you have your figures together. The online return asks only the questions relevant to you, calculates the tax as you go and gives you an immediate acknowledgement. This guide walks through the process.
Online or paper
| Online | Paper (SA100) | |
|---|---|---|
| Deadline for 2025/26 | 31 January 2027 | 31 October 2026 |
| Tax calculated for you | Yes, instantly | HMRC calculates after receipt |
| Acknowledgement | Immediate submission receipt | None unless you ask |
| Suitable for most people | Yes | Only if you cannot file online |
Some returns cannot be filed through HMRC’s own online service and need commercial software, for example certain trust returns or returns with some non-resident or specialist pages.
If you are a sole trader or landlord in Making Tax Digital for Income Tax, you submit your tax return through your MTD software after sending your quarterly updates.
What you need before you start
- your UTR (Unique Taxpayer Reference) and HMRC sign-in details
- your National Insurance number
- employment: P60 (or P45 if you left), P11D for benefits, details of any job expenses
- self-employment: totals of income and allowable expenses, and capital allowance details; see sole trader expenses
- property: rents received and expenses for each property type, and mortgage interest for the finance cost relief
- savings and investments: interest and dividends outside ISAs
- pensions: State Pension and other pensions received; contributions you made to personal pensions
- Gift Aid donations
- capital gains: disposal proceeds, costs and dates, if you sold assets
- Child Benefit received, if the High Income Child Benefit Charge applies
- student loan plan type, if you are repaying through Self Assessment
Step-by-step: filing online
- Sign in to your HMRC online account and choose the Self Assessment return for the right tax year.
- Tailor your return. Answer the opening questions about your income types. The online return then shows only the sections you need, such as employment, self-employment, UK property or capital gains.
- Enter your income, section by section, using totals from your records. For self-employment, businesses with turnover below £90,000 can usually use the short self-employment pages.
- Enter reliefs and allowances: personal pension contributions (gross), Gift Aid, Marriage Allowance, and any losses brought forward.
- Add other information, such as underpaid tax collected through your tax code, or a request to collect a small balance through PAYE.
- Check the tax calculation HMRC shows. Compare it with last year and look for anything unexpected.
- Submit the return and save the submission receipt.
You can save your progress and come back, and you can file as soon as the tax year ends on 5 April. Filing early does not mean paying early, but it tells you what you owe in good time.
After you file
- Payment: pay any balance and your first payment on account by 31 January. You can pay by bank transfer, debit card, or Direct Debit set up in advance. See payments on account and the budget payment plan.
- Refunds: if you have overpaid, claim the repayment in your online account.
- Mistakes: you can amend the return online within 12 months of the 31 January deadline; see amending a Self Assessment return.
Common mistakes
- Forgetting income such as interest on large savings, a one-off sale of shares or income from a second job.
- Entering pension contributions net instead of gross. Personal pension contributions go in at the gross amount, including the 20% tax relief added by the provider.
- Claiming non-allowable expenses, such as personal clothing or the capital cost of equipment as an expense where capital allowances apply.
- Missing the High Income Child Benefit Charge where either partner’s income is over £60,000.
- Leaving it to the last day, when HMRC’s systems and helplines are busiest.
Frequently asked questions
When is the deadline to file my Self Assessment online?
31 January after the end of the tax year: 31 January 2027 for 2025/26.
Can I still file a paper tax return?
Yes, using form SA100, but the deadline is 31 October after the end of the tax year, three months earlier than online.
Do I need software to file my tax return?
Not usually. HMRC’s online service is free. Sole traders and landlords in Making Tax Digital, and some special cases, must use compatible software.
How do I know my return has been received?
Online filing gives an immediate submission receipt with a reference. Keep it in case of any dispute.
Sources
Rates, thresholds and rules in this guide were checked against HMRC and GOV.UK guidance in October 2026. Tax rules change, so check the current position on GOV.UK or with a qualified adviser before acting. This guide is general information, not personal advice.