If you are self-employed, freelance, or have income outside of PAYE, you will need to file a Self Assessment tax return with HMRC every year. Missing the deadline means automatic fines — so here is everything you need to know to get it right.
The process can look intimidating the first time, with unfamiliar abbreviations and a looming deadline, but it is much more manageable once you break it down. Most of the work is simply gathering records you already keep for your work, and the HMRC online service does the calculation for you. The two things that genuinely matter are knowing whether you need to register, and never letting the key dates slip, because the penalties for missing them are automatic and unforgiving.
Who Needs to File a Tax Return?
You must file a Self Assessment tax return if any of the following apply:
- Self-employed — you run your own business or work for yourself
- Freelancer or contractor — paid outside of PAYE by clients
- High earner — your total income exceeds £100,000
- Rental income — you receive rent from property
- Capital gains — you have sold shares, property, or other assets at a profit
- Company directors — you receive director’s fees or benefits
- Trustees — you manage a trust
- Foreign income — you receive income from abroad
Even if you already pay tax through PAYE, you may still need to file if you have additional untaxed income.
Deadlines You Cannot Miss
The tax year runs from 6 April to 5 April the following year. There are two key filing deadlines:
| Deadline | What it means |
|---|---|
| 31 October | Paper tax return deadline |
| 31 January | Online tax return deadline + payment deadline |
Late filing penalties kick in immediately after the 31 January deadline:
- £100 flat fine if you are even one day late
- After 3 months: £10 per day (up to 90 days, so up to £900)
- After 6 months: £300 or 5% of tax owed (whichever is greater)
- After 12 months: another £300 or 5% of tax owed
There are separate penalties for late payment, which accrue interest on top.
Get Your UTR Number
A Unique Taxpayer Reference (UTR) is a 10-digit number that HMRC uses to identify you. You need one before you can file.
How to get a UTR:
- Online: Register at GOV.UK
- By phone: Call HMRC on 0300 200 3310
- By post: Complete form SA1
HMRC posts your UTR within 10 working days (or 21 days if you are abroad). Register as soon as possible — do not wait until January, because you cannot file without it.
Registering for Self Assessment
You must register by 5 October following the end of the tax year in which you first had taxable income that needs reporting.
For example, if you started self-employment in the 2025/26 tax year (ending 5 April 2026), you must register by 5 October 2026.
Steps to register:
- Go to GOV.UK and search “register for Self Assessment”
- Fill in your personal details and income information
- You will receive a UTR by post
- Activate your online account using the activation code
What to Report on Your Tax Return
Your tax return covers all income sources from the tax year. You must report:
Self-Employment Income
All profits from your trade, business, or profession. Report this on the self-employment pages (SA103 or online equivalent).
Rental Income
Gross rental income minus allowable property expenses. Report this on the property pages (SA105 or online equivalent).
Capital Gains
Profits from selling assets such as shares, property (not your main home), or cryptocurrency. Use the capital gains pages.
Dividends
Dividend income from shares, outside of ISAs and pensions. Dividends have their own tax rates.
Interest
Savings interest from bank accounts, bonds, and other investments.
Foreign Income
Income from abroad, including foreign wages, pensions, rental income, or investments.
Expenses You Can Deduct
One of the biggest advantages of self-employment is deducting allowable business expenses to reduce your tax bill.
Common deductible expenses include:
- Office costs: rent, utilities, stationery, phone bills
- Travel: fuel, public transport, parking, vehicle maintenance
- Equipment: laptops, tools, machinery, software
- Professional fees: accountants, solicitors, trade body subscriptions
- Insurance: public liability, professional indemnity
- Marketing: advertising, website costs, business cards
- Training: courses directly related to your work
You cannot deduct:
- Personal living costs (food, clothing, household bills)
- Fines and penalties
- Entertainment expenses (unless for staff)
- Anything not exclusively for business use
Keep receipts and records throughout the year — HMRC can ask for evidence up to six years after filing.
National Insurance for the Self-Employed
Self-employed people pay two types of National Insurance:
Class 2 National Insurance
A flat weekly rate (currently £3.45 per week for 2025/26). You pay this if your profits exceed the Small Profits Threshold. You can voluntarily pay Class 2 to protect your State Pension record even if profits are below the threshold.
Class 4 National Insurance
Calculated as a percentage of your profits:
- 9% on profits between £12,570 and £50,270
- 2% on profits above £50,270
Class 4 is calculated automatically when you file your tax return. You do not pay Class 4 on profits below the Lower Profits Limit.
Payment on Account
If your tax bill exceeds £1,000, HMRC requires advance payments towards the following year’s tax. These are called payments on account.
How it works:
- Each payment is 50% of your previous year’s tax bill
- First payment due 31 January
- Second payment due 31 July
For example, if your 2025/26 tax bill is £4,000, you pay:
- £2,000 on 31 January 2027
- £2,000 on 31 July 2027
If your income is expected to drop, you can apply to reduce payments on account — but if you reduce too much, you will face interest charges.
Filing Online via HMRC
The quickest and easiest way to file is through the HMRC online service on GOV.UK.
What you need:
- A Government Gateway account (create one at GOV.UK)
- Your UTR number
- Income and expense records for the tax year
- Details of any other income (dividends, interest, property, etc.)
The online system guides you through each section step by step. It is simpler than the paper form and automatically calculates your tax.
A useful habit is to file early in January to avoid last-minute queues on the GOV.UK website, since the system can slow down or become busy near the deadline. Filing early also means any refund is processed sooner.
Check Your Tax Code
Your tax code tells your employer how much tax to deduct through PAYE. If your tax code is wrong, you could be overpaying or underpaying.
Common tax codes:
- 1257L — standard personal allowance (£12,570)
- BR — all income taxed at basic rate (20%)
- D0 — all income taxed at higher rate (40%)
Check your tax code on your payslip or via your Personal Tax Account on GOV.UK. Contact HMRC immediately if anything looks wrong.
Worked Example: Self-Employed Designer
Let us walk through a complete example to see how the numbers work.
Scenario: Freelance graphic designer, working from home.
| Item | Amount |
|---|---|
| Gross income (clients) | £50,000 |
| Allowable expenses | £15,000 |
| Taxable profit | £35,000 |
Income Tax Calculation
The first £12,570 is your Personal Allowance (tax-free). The remaining profit is taxed at the basic rate of 20%.
- Personal Allowance: £12,570 → £0 tax
- Remaining: £35,000 - £12,570 = £22,430 × 20% = £4,486 tax
Total income tax: £4,486
National Insurance Calculation
- Class 2: 52 weeks × £3.45 = £179.40
- Class 4: (£35,000 - £12,570) × 9% = £22,430 × 9% = £2,018.70
Total National Insurance: £2,198
Total Tax Bill
| Tax | Amount |
|---|---|
| Income Tax | £4,486 |
| Class 2 NI | £179 |
| Class 4 NI | £2,019 |
| Total | £6,684 |
Payment on Account
Since the tax bill exceeds £1,000, two advance payments are due:
- 31 January: £3,342 (half of £6,684)
- 31 July: £3,342
Plus balancing payment for the previous year if applicable.
A Final Word
Self Assessment feels daunting the first time, but it is a routine annual task that becomes easier each year once you have your UTR, an online account, and a habit of keeping good records. The essentials are simple: register early, gather your income and expenses, file before the deadlines, and set money aside for tax throughout the year so that the January bill never comes as a shock. For straightforward returns the online service handles the calculations for you, while an accountant can be worthwhile once your affairs become more complex. Get the basics right and the whole process becomes just another small, manageable part of running your own affairs.