What is Self-Assessment?
Self-assessment is the system HMRC uses to collect income tax from individuals who are not fully taxed at source. This guide explains who must file, deadlines, and how the process works.
Self-Assessment Explained
Self-assessment is the system through which individuals report their income to HMRC and calculate their own tax liability. It applies to anyone whose tax affairs are not fully settled through PAYE alone. The system was introduced in 1996 and is governed by the Taxes Management Act 1970 as amended.
Under self-assessment, taxpayers are responsible for telling HMRC about their income and gains, filing an accurate return, and paying any tax owed by the relevant deadlines.
Who Needs to File a Self-Assessment Return?
You must register for self-assessment and file a tax return if you:
- Are self-employed as a sole trader with income above £1,000
- Are a partner in a business partnership
- Have untaxed income exceeding £2,500 (such as rental income, tips, or commission)
- Receive income from savings, investments, or dividends above certain thresholds
- Earn more than £150,000 per year (even if taxed through PAYE)
- Need to pay Capital Gains Tax on the disposal of assets
- Are a company director (unless for a non-profit with no pay or benefits)
- Have foreign income that is taxable in the UK
- Are a trustee or executor of an estate
- Receive the High Income Child Benefit Charge (income over £60,000)
If you are employed and have no other income, your employer handles your tax through PAYE and you generally do not need to file a self-assessment return.
Registering for Self-Assessment
You must register by 5 October following the end of the tax year in which you first need to file. For example, if you become self-employed in August 2025 (tax year 2025/26), you must register by 5 October 2026.
Registration can be done online through the HMRC website. You will receive a Unique Taxpayer Reference (UTR) — a 10-digit number used for all self-assessment dealings.
To file online, you also need a Government Gateway user ID.
The Tax Year
The UK tax year runs from 6 April to 5 April. For example, the 2024/25 tax year runs from 6 April 2024 to 5 April 2025. All income and expenses reported on a self-assessment return relate to this period.
Key Deadlines
| Deadline | Date |
|---|---|
| Register for self-assessment | 5 October after the tax year |
| Paper return filing | 31 October following the tax year |
| Online return filing | 31 January following the tax year |
| Payment of balancing tax owed | 31 January following the tax year |
| First payment on account | 31 January following the tax year |
| Second payment on account | 31 July following the tax year |
For the 2024/25 tax year:
- Online filing deadline: 31 January 2026
- Paper filing deadline: 31 October 2025
What to Include on Your Return
Your self-assessment return must declare all sources of income, including:
Employment Income
- Salary, wages, bonuses, and benefits in kind
- Information from your P60 and P11D
Self-Employment Income
- Turnover and allowable expenses
- Capital allowances claimed
- Details of the basis period
Other Income
- Rental income from property
- Dividends and interest
- Pension income
- Foreign income
- Gains from selling assets subject to Capital Gains Tax
Payments on Account
If your tax bill is more than £1,000 and less than 80% of your tax was collected at source, you must make payments on account. These are advance payments towards next year’s tax bill, based on the previous year’s liability.
Each payment on account is 50% of the previous year’s tax bill:
- First payment: 31 January (during the tax year)
- Second payment: 31 July (after the tax year ends)
- Balancing payment: 31 January (following the tax year)
You can apply to reduce payments on account if you expect your income to decrease.
Income Tax Rates
Self-assessment uses the standard income tax rates and bands:
| Band | Taxable Income | Rate |
|---|---|---|
| Personal Allowance | Up to £12,570 | 0% |
| Basic rate | £12,571 to £50,270 | 20% |
| Higher rate | £50,271 to £125,140 | 40% |
| Additional rate | Over £125,140 | 45% |
The personal allowance reduces by £1 for every £2 of income above £100,000, disappearing entirely at £125,140.
National Insurance for the Self-Employed
Self-employed individuals also pay National Insurance contributions through self-assessment:
- Class 2 NICs: Flat rate of £3.45 per week (if profits exceed £12,570)
- Class 4 NICs: 6% on profits between £12,570 and £50,270, plus 2% on profits above £50,270
Penalties for Late Filing and Payment
HMRC imposes automatic penalties for late returns:
| How Late | Penalty |
|---|---|
| 1 day late | £100 fixed penalty |
| 3 months late | £10 per day (up to 90 days, maximum £900) |
| 6 months late | 5% of tax due or £300 (whichever is greater) |
| 12 months late | Further 5% of tax due or £300 (whichever is greater) |
Late payment interest is charged at the Bank of England base rate plus 2.5%. Additional surcharges of 5% apply at 30 days, 6 months, and 12 months.
Making Tax Digital and Self-Assessment
From April 2026, self-employed individuals and landlords with income above £50,000 will need to comply with Making Tax Digital for Income Tax . This requires digital record keeping and quarterly updates to HMRC, replacing the traditional annual return for those affected.
The threshold drops to £30,000 from April 2027.
Record Keeping
You must keep records for at least 5 years after the 31 January filing deadline. Required records include:
- Invoices and receipts for income and expenses
- Bank statements
- P60s and P11Ds from employers
- Dividend vouchers
- Records of any assets bought or sold
Maintaining proper accounting records throughout the year makes the self-assessment process significantly easier and reduces the risk of errors.
Common Mistakes to Avoid
- Missing the deadline — set reminders well in advance
- Forgetting income sources — HMRC receives data from employers, banks, and other sources and will query discrepancies
- Not claiming all allowable expenses — review the full list of tax-deductible expenses
- Mixing personal and business finances — use separate bank accounts
- Ignoring payments on account — budget for these advance payments
- Not registering on time — the 5 October deadline is often overlooked by new businesses
How to File
Self-assessment returns can be filed:
- Online through HMRC’s website or compatible software
- On paper using an SA100 form (earlier deadline applies)
Most taxpayers now file online, which also provides an immediate calculation of tax owed. Commercial accounting software can submit returns directly to HMRC and help track income and expenses throughout the year.