Self Assessment UK: Registration, Filing, Payments on Account and PenaltiesSelf Assessment Tax Return UK — Step-by-Step Guide
How to complete your Self Assessment tax return. Who needs to file, deadlines, what you can claim, and avoiding common mistakes.
Self Assessment can seem daunting, but it’s manageable step by step. Here’s how to get it right.
Read more: See our Self Assessment guide for a complete overview of this topic.
Do You Need to File?
You Must File If You’re…
| Situation | File? |
|---|
| Self-employed (over £1,000 income) | Yes |
| Company director | Yes |
| Partner in a partnership | Yes |
| Income over £150,000 | Yes |
| Rental income | Yes |
| Foreign income | Yes |
| Capital gains over allowance | Yes |
| Receiving Child Benefit, income over £60,000 | Yes |
| Untaxed savings interest/dividends | Yes |
You Don’t Usually Need to File If…
| Situation | File? |
|---|
| Only PAYE employment | No |
| State Pension only | Usually no |
| Tax fully deducted at source | No |
Not Sure?
| Action | Details |
|---|
| Check gov.uk | Tool to check |
| HMRC will tell you | If they think you need to |
| When in doubt | Register anyway |
Key Dates
Tax Year 2025-26
| Date | Event |
|---|
| 6 April 2025 | Tax year starts |
| 5 April 2026 | Tax year ends |
| 5 October 2026 | Register for Self Assessment (if new) |
| 31 October 2026 | Paper return deadline |
| 31 January 2027 | Online return deadline |
| 31 January 2027 | Pay tax owed |
| 31 July 2027 | Second payment on account (if applicable) |
Payments on Account
| What | Advance payments toward next year’s tax |
|---|
| When | If tax bill over £1,000 |
| Amount | 50% of previous year’s bill |
| Due | 31 January and 31 July |
| First year | No payments on account |
Before You Start
Register (If New)
| Step | Action |
|---|
| 1 | Register at gov.uk |
| 2 | Receive UTR (Unique Taxpayer Reference) |
| 3 | Wait for activation code |
| 4 | Set up Government Gateway |
| Timeline | Allow 2-3 weeks |
Gather Documents
| Document | For |
|---|
| P60 | Employment income |
| P45 | If left job during year |
| P11D | Benefits in kind |
| Bank statements | Interest earned |
| Dividend vouchers | Share dividends |
| Rental accounts | Property income |
| Business records | Self-employment |
| Pension contributions | Tax relief claims |
| Gift Aid receipts | Charity donations |
Completing the Return
The Sections
| Section | Who Completes |
|---|
| Personal details | Everyone |
| Employment | If employed |
| Self-employment | If self-employed |
| Partnership | If in partnership |
| UK property | If rental income |
| Foreign income | If overseas income |
| Capital gains | If sold assets |
| Additional info | Various |
Employment Section
| Enter | Details |
|---|
| Employer name | From P60 |
| Pay | Total before tax |
| Tax deducted | Amount already paid |
| Benefits | From P11D |
Self-Employment Section
| Calculate | Details |
|---|
| Total income | All business earnings |
| Allowable expenses | See below |
| Profit | Income minus expenses |
| This is what’s taxed | |
Expenses You Can Claim
Self-Employed Expenses
| Category | Examples |
|---|
| Office costs | Stationery, phone, software |
| Travel | Business mileage, public transport |
| Clothing | Uniforms, protective equipment |
| Staff costs | Wages, subcontractors |
| Stock | Goods for resale |
| Premises | Rent, utilities, repairs |
| Advertising | Marketing, website |
| Professional fees | Accountant, legal |
| Insurance | Business insurance |
| Training | Courses for current trade |
Working From Home
| Method | How It Works |
|---|
| Simplified | £6/week flat rate |
| Actual costs | Proportion of bills |
| Calculation | Hours worked × rate or % of home |
Vehicle Expenses
| Method | Rate |
|---|
| Simplified (mileage) | 45p/mile first 10,000, then 25p |
| Actual costs | Fuel, insurance, repairs × business % |
Expenses You Can’t Claim
| Not Allowable | Why |
|---|
| Personal expenses | Not business |
| Entertainment | HMRC rule |
| Fines | Not allowed by law |
| Your own wages | That’s the profit |
| Living costs | Personal |
Property Income
What to Include
| Income | Include |
|---|
| Rent received | All rent |
| Service charges | If you keep them |
| Insurance payouts | For lost rent |
Property Expenses
| Deductible | Examples |
|---|
| Letting agent fees | Management costs |
| Repairs | Maintenance, fixing |
| Insurance | Landlord insurance |
| Ground rent | Leasehold costs |
| Council Tax | If you pay it |
| Utilities | If you pay them |
| Mortgage interest | 20% tax credit only |
Mortgage Interest (Restricted)
| Current Rule | Details |
|---|
| No deduction | From rental income |
| Instead | 20% tax credit |
| Higher rate taxpayers | Pay more tax than before |
Common Mistakes to Avoid
Filing Errors
| Mistake | Consequence |
|---|
| Missing deadline | £100+ penalties |
| Wrong UTR | Return rejected |
| Forgetting income | Penalties, interest |
| Over-claiming expenses | Investigation risk |
| No records | Can’t prove claims |
Income Often Forgotten
| Income Source | Remember |
|---|
| Bank interest | Even if small |
| Dividends | Outside ISA |
| Occasional freelance | Over £1,000 |
| Rental rooms | Over Rent-a-Room limit |
| Crypto gains | Capital gains section |
Paying Your Tax Bill
How to Pay
| Method | Details |
|---|
| Direct Debit | Set up via HMRC |
| Online banking | Use HMRC account details |
| BACS | Bank transfer |
| Credit card | Via third party (fee) |
| Budget payment plan | Monthly payments |
Can’t Pay?
| Option | Details |
|---|
| Time to Pay | HMRC payment plan |
| Call HMRC | Before deadline |
| Be honest | About circumstances |
| Set up online | For debts under £30,000 |
After Filing
Keep Records
| How Long | 5 years after 31 January deadline |
|---|
| What | All receipts, bank statements, invoices |
| Why | HMRC can investigate |
| How | Digital or paper |
If You Made a Mistake
| Timeframe | Action |
|---|
| Before deadline | Amend and resubmit |
| After deadline | Contact HMRC |
| Within 12 months | Amend online |
| After 12 months | Write to HMRC |
Summary: Self Assessment Checklist
Before Starting
| Task | Done |
|---|
| Registered for Self Assessment | ☐ |
| Have UTR number | ☐ |
| Government Gateway access | ☐ |
| All documents gathered | ☐ |
Income to Report
| Source | Amount |
|---|
| Employment | £ |
| Self-employment | £ |
| Rental | £ |
| Dividends | £ |
| Interest | £ |
| Other | £ |
Expenses Claimed
| Category | Amount |
|---|
| Business expenses | £ |
| Property expenses | £ |
| Pension contributions | £ |
| Gift Aid | £ |
Key Deadlines
| Date | What |
|---|
| 5 October | Register if new |
| 31 January | File online + pay tax |
| 31 July | Second payment on account |
After Filing
| Task | Done |
|---|
| Saved confirmation | ☐ |
| Paid tax owed | ☐ |
| Set aside for next year | ☐ |
| Records stored safely | ☐ |
Self Assessment is straightforward once you’ve done it once. Keep good records throughout the year, gather everything before you start, and don’t leave it until the last minute. Most people can complete it themselves using HMRC’s online system.
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