Self Assessment is how you tell HMRC about income that hasn’t been taxed at source. If you’re self-employed, a landlord, or have investment income, you’ll almost certainly need to file a tax return. Getting it wrong can mean penalties — but filing correctly saves you money.
Who Needs to File a Self Assessment Tax Return?
You must file a Self Assessment return if any of the following apply:
Mandatory Filers
| Category | Why You Must File |
|---|---|
| Self-employed or a sole trader | Income not taxed through PAYE |
| Company directors | Dividends and salary over certain limits |
| Landlords with rental income | Property income over £2,500 (or lower if HMRC requires it) |
| High earners (over £150,000) | Income above the additional rate threshold |
| People with investment income | Dividends, interest, or capital gains over £10,000 |
| People with income from abroad | Foreign income not taxed in the UK |
| Trustees | Income held in trust |
| Executors or administrators | Income from estates during administration |
Optional Filers
You may also want to file if:
- You want to claim a tax refund (e.g., overpaid through PAYE)
- You’re not sure if you need to file — check with HMRC
- You want to claim certain tax reliefs (Gift Aid, pension relief)
Your UTR Number
Every person registered for Self Assessment has a Unique Taxpayer Reference (UTR) — a 10-digit number. You’ll need this for all correspondence with HMRC.
How to get a UTR:
- Register for Self Assessment online at gov.uk
- HMRC will post your UTR within 10 working days (28 days if overseas)
- You can also find it on previous tax returns or correspondence from HMRC
Keep your UTR safe — you’ll need it every time you file or contact HMRC.
Deadlines You Must Not Miss
Missing a deadline can result in automatic penalties. Here are the key dates:
Filing Deadlines
| Deadline | Type | Applies To |
|---|---|---|
| 31 October (paper return) | Paper filing | Anyone filing by post |
| 31 January (online return) | Online filing | Everyone filing online |
Payment Deadlines
| Deadline | What’s Due |
|---|---|
| 31 January | Balancing payment for previous tax year + first payment on account |
| 31 July | Second payment on account |
Example Timeline for 2026/27
| Date | What’s Due |
|---|---|
| 6 April 2026 | Tax year starts |
| 5 April 2027 | Tax year ends |
| 31 July 2027 | Second payment on account |
| 31 October 2027 | Paper return deadline |
| 31 January 2028 | Online return deadline + balancing payment + first payment on account |
What Records You Need
HMRC expects you to keep records throughout the year. Here’s what to gather:
For All Filers
| Record | Why It’s Needed |
|---|---|
| P60 | Shows total income and tax deducted through PAYE |
| P45 | If you left a job during the year |
| P11D or benefit statements | Employer-provided benefits (company car, private medical) |
| Bank statements | Interest earned, dividends received |
| Pension statements | Contributions made, tax relief claimed |
| Investment statements | Capital gains, dividends, interest |
For Self-Employed People
| Record | Why It’s Needed |
|---|---|
| Sales and invoices | Total turnover |
| Purchase receipts and invoices | Allowable expenses |
| Bank statements | All income and expenses |
| Mileage records | Business travel claims |
| Home office records | Proportion of home used for business |
| Professional subscriptions | Allowable if related to your work |
For Landlords
| Record | Why It’s Needed |
|---|---|
| Rental income records | Total rent received |
| Mortgage statements | Interest paid (20% tax credit) |
| Property expenses | Repairs, maintenance, insurance |
| Agent fees | Management costs |
| Stamp Duty Land Tax | Purchase costs |
| Solicitor and surveyor fees | Transaction costs |
How Long to Keep Records
Keep records for at least five years after the 31 January deadline for the tax year. For example, for the 2026/27 tax year, keep records until at least 31 January 2033.
How to Register for Self Assessment
Step 1: Check if You Need to Register
You must register by 5 October after the end of the tax year in which you first had taxable income that wasn’t taxed at source.
Example: If you became self-employed in the 2026/27 tax year (ending 5 April 2027), you must register by 5 October 2027.
Step 2: Register Online
- Go to gov.uk and search for “Register for Self Assessment”
- Choose the right category: self-employed, landlord, or other
- Complete the online form with your details
- HMRC will send your UTR by post
Step 3: Activate Your Account
Once you receive your UTR, you can activate your online account:
- Go to gov.uk and search for “Activate your Self Assessment account”
- Enter your UTR and follow the activation steps
- Set up your Government Gateway credentials
How to File Online
Step 1: Log In to Your Government Gateway Account
Go to gov.uk and log in to your Self Assessment account using your UTR and password.
Step 2: Complete the Return
The online form guides you through each section:
| Section | What to Enter |
|---|---|
| Employment | Salary, benefits, employment expenses |
| Self-employment | Turnover, expenses, profit |
| Landlord | Rental income, property expenses |
| Interest | Bank and building society interest |
| Dividends | Dividend income |
| Capital gains | Gains from selling assets |
| Pension contributions | Contributions made, tax relief |
| Gift Aid | Charitable donations |
| Blind person’s allowance | If applicable |
Step 3: Check Your Tax Calculation
The system will automatically calculate your tax bill based on what you’ve entered. Review it carefully — check that your income, expenses, and reliefs are correct.
Step 4: Submit and Pay
Once you’re happy with the return:
- Submit it electronically
- Make payment using one of HMRC’s accepted methods:
- Direct debit
- Online bank transfer (Faster Payments)
- Debit or credit card
- Tax code adjustment (if you have a PAYE job)
Payments on Account
If your tax bill for the previous year was more than £1,000 and less than 20% was collected at source (e.g., through PAYE), HMRC will require you to make payments on account for the current year.
How Payments on Account Work
| Payment | Date | Amount |
|---|---|---|
| First payment on account | 31 January | 50% of previous year’s tax bill |
| Second payment on account | 31 July | 50% of previous year’s tax bill |
| Balancing payment | 31 January (next year) | Difference between your actual bill and payments on account |
Example
| Item | Amount |
|---|---|
| 2025/26 tax bill | £6,000 |
| First payment on account (31 Jan 2027) | £3,000 |
| Second payment on account (31 Jul 2027) | £3,000 |
| 2026/27 actual tax bill | £8,000 |
| Balancing payment (31 Jan 2028) | £2,000 |
Reducing Payments on Account
If you think your tax bill will be lower this year, you can apply to reduce payments on account. You can do this online through your Self Assessment account. Be careful — if you reduce too much and underpay, you’ll owe interest on the shortfall.
Penalties for Late Filing and Payment
HMRC takes deadlines seriously. Here’s what you’ll face if you miss them:
Filing Penalties
| Penalty | When It Applies |
|---|---|
| £100 fixed | Return filed up to 3 months late |
| £10 per day | 3 to 6 months late (up to 90 days = £900) |
| £300 or 5% of tax due (whichever is higher) | 6 to 12 months late |
| £300 or 5% of tax due (whichever is higher) | Over 12 months late (another penalty) |
Total possible penalty for a year: Over £1,600 plus the tax you owe.
Payment Penalties
| Penalty | When It Applies |
|---|---|
| 3.25% surcharge | Tax not paid by 31 January |
| Additional 3.25% | Tax still unpaid after 6 months |
| Daily penalty of £10 | Up to 90 days |
Interest
HMRC charges interest on late payments at the official rate (currently around 7%). This compounds daily, so the longer you delay, the more you owe.
How to Reduce Your Tax Bill
1. Claim All Allowable Expenses
Self-employed people can deduct business expenses from their income:
| Expense | Allowable? |
|---|---|
| Office costs (rent, utilities, equipment) | Yes |
| Travel and vehicle costs | Yes (business miles only) |
| Stock and raw materials | Yes |
| Marketing and advertising | Yes |
| Professional fees (accountant, solicitor) | Yes |
| Insurance | Yes |
| Phone and internet (business use) | Yes (proportion) |
| Training related to your work | Yes |
| Bank charges and interest | Yes |
| Clothing (protective/uniform only) | Yes |
Not allowable: General clothing, commuting costs, fines, entertaining clients.
2. Claim Home Office Costs
If you work from home, you can claim a proportion of your household costs:
Simplified method:
| Hours Worked from Home | Monthly Allowance |
|---|---|
| 25-50 hours | £10 |
| 51-100 hours | £18 |
| 101+ hours | £26 |
Actual cost method: Calculate the proportion of your home used for business (e.g., a spare bedroom used as an office). Claim that percentage of rent, mortgage interest, utilities, and insurance.
3. Make Pension Contributions
Pension contributions reduce your taxable income. A personal pension contribution gets basic rate relief automatically — higher rate taxpayers can claim additional relief through Self Assessment.
Example: A £10,000 pension contribution for a 40% taxpayer reduces their tax bill by £2,500 (the additional relief on top of the basic rate relief already claimed).
4. Use Your ISA Allowance
Investments within an ISA are free from capital gains tax and income tax. Maximise your £20,000 ISA allowance each year.
5. Claim Marriage Allowance
If you earn less than £12,570 and your spouse earns between £12,571 and £50,270, you can transfer £1,260 of your personal allowance to them. This saves up to £252 per year.
6. Time Your Income
If you have control over when you receive income:
- Delay invoicing until after 6 April to defer income into the next tax year
- Bring forward expenses into the current tax year
- Consider the timing of bonuses or dividends
Common Self Assessment Mistakes
- Missing the 31 January deadline — even if you owe no tax, you must file
- Not registering in time — register by 5 October or face penalties
- Forgetting to include all income — HMRC checks against records from banks, employers, and Land Registry
- Claiming personal expenses as business — HMRC investigates excessive claims
- Not keeping records — you must be able to support every figure on your return
- Ignoring payments on account — these are not optional if you owe over £1,000
- Filing a paper return when you could file online — online is easier and gives you an extra three months
Where to Get Help
| Resource | What It Offers |
|---|---|
| HMRC Self Assessment helpline | 0300 200 3310 — general queries |
| GOV.UK Self Assessment guide | Step-by-step online filing instructions |
| HMRC online tutorials | Walkthrough of the online form |
| Tax calculation tool | Estimate your bill before filing |
| Professional accountant | Help with complex returns |
| Tax Aid / TaxHelp for Older People | Free advice for low-income taxpayers |
Filing Self Assessment doesn’t have to be stressful. Keep good records throughout the year, know your deadlines, and claim everything you’re entitled to. The earlier you file, the sooner you know what you owe — and the more time you have to plan for the payment.