Self Assessment Tax Return Guide UK (2026/27) – Step-by-Step
A beginner-friendly Self Assessment guide: who must file, the key dates, exactly how to do it step by step, and how much tax and National Insurance you will pay.
Who needs to file a Self Assessment tax return?
You normally need to file a Self Assessment tax return if any of the following apply in 2026/27:
- You are self-employed with profits over £1,000
- You earned more than £100,000 in the tax year
- Your rental income from property was above £2,500
- You need to pay Capital Gains Tax on selling assets
- You receive foreign income or untaxed income such as savings interest above your allowance
- Your income is over £50,000 and you or your partner get Child Benefit
HMRC sends a notice to file if you are registered. If you are unsure, use the official gov.uk checker.
Key Self Assessment dates (2026/27)
| Deadline | What you must do |
|---|---|
| 5 October 2027 | Register for Self Assessment if new |
| 31 October 2027 | Paper tax return deadline |
| 31 January 2028 | Online return + payment deadline |
| 31 July 2028 | Second payment on account (if applicable) |
Step-by-step: how to do your Self Assessment
- Register for Self Assessment by 5 October and get your Unique Taxpayer Reference (UTR).
- Record your income and expenses – keep receipts and track everything you can legitimately claim.
- Gather your documents – bank interest, dividends, P60s, rental income, and any self-employment records.
- Complete the return online at gov.uk – it walks you through each section.
- Check your numbers – mistakes are the most common reason for HMRC enquiries.
- Submit and pay by 31 January. Set up a direct debit to spread payments.
How much tax and National Insurance will you pay?
Your Income Tax is calculated on your profit (income minus allowable expenses) above the £12,570 personal allowance, at 20%, 40% or 45% depending on your income band.
As a self-employed person you also pay Class 2 and Class 4 National Insurance on your profits. Getting this right is where most people make mistakes – our free calculator does it for you in seconds.
Work out your National Insurance instantly
Enter your annual profit and get your Class 2 and Class 4 liability with official 2026/27 HMRC rates.
Calculate your NI →Common mistakes and penalties
- Filing late – £100 penalty from day one, then more after 3, 6 and 12 months.
- Forgetting income – missing rental or savings income is a common trigger for enquiries.
- Wrong tax code – check your code; a mistake can mean overpaying or underpaying all year.
- Not paying on time – interest is charged on late payments plus penalties.
- Ignoring National Insurance – it is part of your bill, not optional.
Self Assessment FAQs
Who needs to file a Self Assessment tax return?
Self-employed people with profits over £1,000, anyone earning over £100,000, landlords with rental income above £2,500, and anyone who needs to pay Capital Gains Tax. HMRC tells you if you must register.
When is the Self Assessment deadline?
Register by 5 October. Paper returns by 31 October; online returns and payment by 31 January after the tax year ends.
How much tax will I pay as a self-employed person?
Income Tax on profit above £12,570 (20/40/45%) plus Class 2 and Class 4 National Insurance. Use our National Insurance calculator for the NI part.
What is the penalty for filing late?
£100 after one day, then £10 a day after 3 months (up to £900), with larger penalties at 6 and 12 months plus interest.
Related guides: Inheritance tax gifts: 7-year rule explained · How to reduce Capital Gains Tax · Tools: NI Calculator · Overpaid Tax Calculator