Self assessment · Registration guide · UK
How to register for self-assessment UK: step-by-step guide.
Source: gov.uk/register-for-self-assessment.
Written by Mehmood Rajoka
Managing Partner, RR Accountants · IFA-supervised practice
Who needs to register?
You must register for self-assessment if you have untaxed income that HMRC has not collected via PAYE. This includes rental income above £1,000, self-employment income above £1,000, dividends above the £500 dividend allowance, income above £100,000, and director income that isn't fully taxed at source. The registration deadline is 5 October following the tax year in which the income arose. See gov.uk/self-assessment-tax-returns.
How to register and file — step by step
Register with HMRC
Choose the right registration route by 5 October: CWF1 (self-employed / sole trader), SA1 (directors, landlords, other untaxed income), or SA401 (partners). Start at gov.uk/register-for-self-assessment with your National Insurance number to hand.
Get your UTR
HMRC posts your 10-digit Unique Taxpayer Reference. Allow ~10 working days. You can't file — or authorise an accountant — without it, which is why leaving registration until October is risky.
Set up a Government Gateway account
Create a Government Gateway user ID and password. You'll need it to file every year and to authorise an agent. Keep the credentials safe.
Gather your records
All income (P60, self-employment, dividends, rental, savings, pensions, foreign) and the expenses and reliefs you'll claim (including pension contributions). Organised records make filing fast and reduce the chance of an enquiry.
Complete the SA100
File the SA100 main return online through your Government Gateway, with supplementary pages as needed (self-employment, property, capital gains). Submit and keep a copy — generally for at least 5 years after the 31 January deadline.
Note your tax bill
The system calculates what you owe — including any first payment on account. Pay by 31 January following the end of the tax year. For 2025/26: pay by 31 January 2027.
Sources: gov.uk/register-for-self-assessment, gov.uk/self-assessment-tax-returns, LITRG.
FAQs
Who needs to register for self-assessment?
You must register for self-assessment if in the previous tax year you: earned income from property rental; were self-employed with income above £1,000; earned income not taxed at source above £2,500; received dividends and are a higher or additional-rate taxpayer; had income above £100,000; were a company director with untaxed income; or received any other untaxed income. If HMRC has sent you a notice to file, you must file even if you believe you don't need to. See gov.uk/self-assessment-tax-returns.
When is the deadline to register for self-assessment?
If you need to register for the first time, the deadline is 5 October following the end of the tax year in which the income arose. For 2025/26 income (which started 6 April 2025): register by 5 October 2026. Missing this deadline can result in a late registration penalty.
What is a UTR number?
A UTR (Unique Taxpayer Reference) is a 10-digit reference number that HMRC issues when you register for self-assessment. Every registered taxpayer has one. You need your UTR to file your return, set up a HMRC online account, and authorise an accountant to act on your behalf. HMRC sends it by post — it typically takes 10 working days after registration.
How do I register for self-assessment online?
Register via HMRC's Government Gateway at gov.uk/register-for-self-assessment. You will need: your National Insurance number; personal contact details; details of your income sources; and a Government Gateway user ID (or to create one). Different registration routes exist for self-employed (CWF1), non-self-employed including directors and landlords (SA1), and partners (SA401).
How long does it take to register?
Online registration takes around 10–15 minutes. HMRC then sends your UTR by post — typically within 10 working days. Allow extra time around the October deadline period when volumes are high. Do not leave registration until October if you need the UTR to authorise an accountant or set up digital services — start earlier.
Do landlords need to register for self-assessment?
Yes — if your rental income exceeds £1,000 per year, you must register for self-assessment and declare it. Even if your profit after expenses is below the personal allowance, you must still file if your gross rental income exceeds the threshold. MTD for Income Tax adds a further obligation from April 2026 for landlords above £50,000 gross.
Self Assessment, done properly, on time, by an accountant.
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Chartered practice files your 2025/26 return, applies every allowance, flags payments on account, and answers HMRC enquiries. From £150+VAT. 48-hour question response.
See Self Assessment serviceFree tool
Estimate my tax bill first
The Self Assessment tax calculator gives you a 2025/26 estimate — useful before you decide whether to file yourself or hand it over.
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See SmartBooksMore from the Self Assessment series
Companion guides covering who needs to file, deadlines, penalties, payments on account, and director-specific rules.
Do you need to file a return?
The triggers that pull you into Self Assessment — side hustles, dividends, rental, high income.
Deadlines and key dates
5 October to register, 31 October paper, 31 January online + payment, 31 July second POA.
Penalties (and how they escalate)
The automatic £100, the £10/day phase, and the 5% late-payment surcharges.
Payments on account explained
Why your first January bill can feel 50% bigger than expected — and how SA303 works.
Self Assessment for directors
Reporting dividends, the £500 allowance, and 2026/27 dividend tax rates.
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About the author
Mehmood Rajoka, Managing Partner, RR Accountants
Managing Partner at RR Accountants — a UK practice supervised by the Institute of Financial Accountants. Specialist focus on UK landlord and property tax, Self Assessment for directors, and MTD for Income Tax. RR Accountants serves clients across nine physical UK offices plus Glasgow service area.
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This guide is general information about UK tax rules. It is not personal tax advice. For advice tailored to your situation, speak to a regulated UK accountant. All figures verified against gov.uk as of . Self Assessment rules and rates change each year — re-check primary sources before acting.