How do I register for Self Assessment?

Register online at gov.uk/log-in-file-self-assessment-tax-return or call HMRC on 0300 200 3310. You need a Government Gateway account. Registration must be completed by 5 October in the tax year after you started self-employment.

What is the Self Assessment deadline?

Online returns must be submitted by 31 January following the end of the tax year. Paper returns must be filed by 31 October. Tax owed must also be paid by 31 January.

Self Assessment Tax Return UK 2025/26 | Complete Step-by-Step Guide

What Is Self Assessment?

Self Assessment is HMRC's system for collecting income tax from people whose tax is not automatically deducted through PAYE. If you are self-employed, a partner in a business, have significant investment income or other untaxed earnings, you almost certainly need to complete an annual Self Assessment tax return.

The return asks for your total income from all sources — self-employment, employment, property, dividends, savings interest — calculates the tax owed, and allows you to deduct allowable expenses and claim reliefs. HMRC uses the information to produce a tax calculation; you then pay any balance due by 31 January.

Who Needs to Register for Self Assessment?

You must register for Self Assessment if any of the following apply to you:

✓You are self-employed as a sole trader and earn more than £1,000 in a tax year (above the trading allowance)
✓You are a partner in a business partnership
✓Your income is over £150,000 (even if all through PAYE)
✓You have untaxed income over £2,500 — including rental, dividends, savings or foreign income
✓You need to claim certain tax reliefs such as Gift Aid on large donations
✓You are a company director receiving untaxed income

To learn how to get started, see our guide on how to register as self-employed in the UK.

How to Register for Self Assessment

Registration is online at the HMRC website. You will need a Government Gateway user ID — create one if you do not already have one. Once registered, HMRC sends your Unique Taxpayer Reference (UTR) by post within 10 working days. You need your UTR to file your return.

The registration deadline is 5 October following the end of the tax year in which you became self-employed. Miss it and HMRC may impose a late-registration penalty, though penalties for first-year lateness are rare if there is a reasonable excuse.

The HMRC online portal for filing is the Self Assessment Gateway, accessed through your Government Gateway account.

What Is the SA100 Form?

The SA100 is the main Self Assessment tax return form. It covers personal details, income from employment, self-employment (via the supplementary SA103 pages), property, dividends and other sources. You do not typically complete a paper SA100 — the equivalent fields appear in HMRC's online return or in tax software.

For the self-employment section, you complete either the short version (turnover under £85,000) or the full version. If you operate through a limited company, you file a CT600 corporation tax return separately — see Ltd company tax calculator for an overview.

Step-by-Step: Completing Your Return

Filing online through HMRC's portal is straightforward once you have your records. Follow these steps:

Log in at the HMRC Self Assessment Gateway using your Government Gateway credentials.
Start a new return for the relevant tax year (6 April – 5 April).
Enter your income from all sources — self-employment, employment, savings, dividends, rental.
Enter your allowable business expenses on the SA103 pages to arrive at your net profit.
Claim any reliefs — pension contributions, gift aid, losses carried forward.
Review HMRC's tax calculation. Check it makes sense against your records.
Submit the return and note your payment reference.
Pay any tax owed by 31 January.

Self Assessment Deadlines and Penalties

Deadline	Action	Penalty if Missed
5 October	Register for Self Assessment	Potential fine (uncommon for first year)
31 October	Paper return submission	£100 automatic penalty
31 January	Online return + tax payment	£100 penalty + daily charges after 3 months
31 July	Second payment on account	Interest on unpaid amount

Source: HMRC / gov.uk · Rates correct for 2025/26 tax year.

Penalties increase significantly if returns remain unfiled: £10 per day after 3 months (up to £900), plus further charges at 6 and 12 months. If you genuinely cannot file on time, contact HMRC early — reasonable excuse claims can sometimes waive initial penalties.

Does Universal Credit Affect Self Assessment?

If you receive Universal Credit, you must report any self-employment income to the Department for Work and Pensions (DWP) monthly. Universal Credit itself is not taxable income, so you do not normally declare it on your Self Assessment return. However, if HMRC queries your declared income against DWP records, discrepancies can trigger an investigation. See our dedicated guide on declaring Universal Credit on Self Assessment for the exact rules.

Frequently Asked Questions

FAQ

What is the meaning of Self Assessment?+

Self Assessment is the UK tax system requiring individuals to report their own income and calculate the tax owed, rather than having it automatically deducted. It is used by the self-employed, company directors, high earners and those with multiple income sources.

How do I find my Self Assessment gateway login?+

Access the Self Assessment Gateway at gov.uk using your Government Gateway user ID and password. If you have forgotten your credentials, HMRC provides account recovery options. Your UTR (Unique Taxpayer Reference) is needed alongside your Government Gateway login.

What happens if I miss the Self Assessment deadline?+

An automatic £100 penalty is charged from day one of being late, regardless of whether you owe any tax. After 3 months, daily penalties of £10 start (up to £900). Penalties escalate further at 6 and 12 months. Interest also accrues on any unpaid tax from 31 January.

Can I file Self Assessment if I have no tax to pay?+

Yes — if you are registered for Self Assessment, you must file a return even if your income is below the personal allowance and you owe nothing. Failing to file still attracts penalties. You can apply to HMRC to be deregistered if you are no longer self-employed.

What does 'register for Self Assessment' mean?+

It means formally telling HMRC that you need to file a Self Assessment tax return. Once registered, you receive a Unique Taxpayer Reference (UTR) and are expected to file annually until you deregister. You must do this by 5 October following the tax year you became self-employed.