Self Assessment is HMRC’s system for collecting income tax from people whose tax is not fully collected through PAYE. If you are self-employed, have untaxed income, or meet other criteria, you must file a tax return each year — and getting it right can save you money while keeping you on the right side of HMRC.
Who Needs to Register
You must register for Self Assessment and file a tax return if you:
- Are self-employed and earned over £1,000 in the tax year
- Are a company director (excluding non-profit organisations)
- Earned £150,000 or more (even if all via PAYE)
- Had untaxed income — rental income, foreign income, tips, commission
- Need to claim tax relief on employment expenses, pension contributions, or Gift Aid
- Are a partner in a business partnership
- Received child benefit and you or your partner earned over £60,000 (High Income Child Benefit Charge)
Key Deadlines
| Action | Deadline |
|---|---|
| Register for Self Assessment | By 5 October after the tax year ends |
| Paper tax return | 31 October |
| Online tax return | 31 January |
| Pay tax owed | 31 January |
| First payment on account | 31 January |
| Second payment on account | 31 July |
Payments on Account
If your tax bill exceeds £1,000 and less than 80% of your tax is collected at source, HMRC requires payments on account — advance payments towards next year’s bill. Each payment is 50% of the previous year’s tax bill.
What You Need to File
Gather the following before starting your return:
- UTR (Unique Taxpayer Reference) — your 10-digit HMRC reference
- P60 or P45 — if employed during the tax year
- Self-employment income and expenses — total income and allowable expenses
- Bank and savings interest — statements from all accounts
- Dividend income — dividend vouchers or statements
- Rental income and expenses — if you are a landlord
- Pension contributions — to claim additional tax relief (higher/additional rate taxpayers)
- Gift Aid donations — to claim tax relief
- Capital gains — profits from selling assets (shares, property, crypto)
- Foreign income — any income from overseas sources
Filing Your Return Online
Step-by-Step
- Log in to your HMRC online account (or create one)
- Start your return for the relevant tax year
- Complete each section — the system asks which sections apply to you
- Employment income — pre-filled from your employer’s data (check it is correct)
- Self-employment — enter turnover, expenses, and profit
- Other income — savings, dividends, rental, foreign
- Tax reliefs — pension contributions, Gift Aid, marriage allowance
- Review and submit — check the calculated tax figure
- Pay — via direct debit, bank transfer, or debit card
Self-Employment Section
The simplified format for small businesses:
| Field | What to Enter |
|---|---|
| Turnover | Total business income (not profit) |
| Allowable expenses | Total deductible costs |
| Net profit | Turnover minus expenses (calculated) |
Use the self-employment tax calculator to estimate your tax bill before filing.
Reducing Your Tax Bill
Claim All Allowable Expenses
If self-employed, ensure you claim every legitimate business expense — office costs, travel, phone, insurance, professional subscriptions, and more. See our allowable expenses guide.
Use Your Personal Allowance Fully
Everyone has a £12,570 personal allowance (2025/26). Income up to this amount is tax-free.
Pension Contributions
Contributions to your pension receive tax relief. Basic rate relief is added automatically; higher and additional rate taxpayers claim the extra relief through Self Assessment.
Marriage Allowance
If you or your partner earns less than £12,570 and the other is a basic rate taxpayer, you may be able to transfer £1,260 of personal allowance — saving up to £252/year. See our marriage allowance guide.
Gift Aid
Tax relief on charitable donations is claimed through Self Assessment. Higher and additional rate taxpayers reclaim the difference between their rate and the basic rate.
Capital Gains Tax Annual Exempt Amount
The first £3,000 of capital gains (2025/26) is tax-free. Use this each year before it expires.
Common Mistakes to Avoid
- Missing the deadline — set a reminder for December at the latest
- Forgetting income sources — HMRC receives data from banks, employers, and platforms — discrepancies trigger enquiries
- Overclaiming expenses — only claim genuine business expenses with supporting records
- Not keeping records — maintain receipts and records for at least 5 years (6 years for self-employed)
- Ignoring payments on account — budget for the July payment on account, not just the January bill
- Not claiming reliefs — many people miss pension relief, marriage allowance, and employment expenses
What If You Cannot Pay?
If you cannot pay your tax bill in full by 31 January:
- Set up a payment plan — HMRC’s Time to Pay service lets you spread payments over up to 12 months
- Contact HMRC early — the sooner you call, the more flexible they tend to be
- Do not ignore it — interest and penalties accumulate; HMRC has strong collection powers
You can set up a payment plan online if your bill is under £30,000 and you are within 60 days of the payment deadline.