How to file a Self Assessment tax return

What Self Assessment is

Self Assessment is the system HMRC uses for taxpayers to report income and gains that are not fully covered by tax deducted at source. Most employees pay tax through PAYE on their wages, but self-employed people, landlords, company directors with untaxed income, and many others report their affairs on an annual tax return instead.

The tax year runs from 6 April to 5 April. A return for the year ending 5 April 2026 covers income received between 6 April 2025 and 5 April 2026. HMRC uses the figures on the return to work out how much Income Tax and National Insurance is owed — or whether a refund is due.

Filing is separate from paying. Submitting the return tells HMRC what happened in the year; payment settles the bill HMRC calculates afterwards. Both have deadlines, and missing either can lead to penalties or interest.

Who needs to file

Not everyone in the UK files Self Assessment. HMRC requires a return when untaxed income crosses certain thresholds or when specific circumstances apply.

Common reasons a taxpayer may need to file include:

• Self-employment profits above the £1,000 trading allowance
• Rental income above the £1,000 property allowance (or when the allowance is not claimed)
• Dividends or savings interest above the personal allowance for those sources
• Income from abroad that is not taxed in the UK
• Capital gains above the annual exempt amount
• Being a company director who received income not taxed through PAYE
• HMRC sending a notice to file a return (form SA316)

Taxpayers who only have PAYE income and no other untaxed sources often do not need a return. HMRC may still send a notice to file if records suggest otherwise.

Anyone unsure whether filing is required can check the official guidance on Self Assessment tax returns or use HMRC's online tools after signing in.

Registering for Self Assessment

Taxpayers who have never filed before need to register before HMRC will accept a return. Registration is not automatic when someone starts working for themselves.

The registration route depends on the reason for filing:

• Self-employed — register as self-employed through HMRC's online service
• Not self-employed but need a return — register for Self Assessment only
• Partners in a partnership — the partnership registers separately; each partner also registers individually

HMRC expects newly self-employed people to register by 5 October after the end of the tax year in which they started trading. For example, someone who began trading in June 2025 (during the 2025–26 tax year) would need to register by 5 October 2026.

After registering, HMRC posts a letter containing the Unique Taxpayer Reference (UTR). This ten-digit number identifies the taxpayer in all Self Assessment dealings. It can take up to 10 working days to arrive. Without a UTR, online filing is not possible.

Registration also triggers an activation code for the Government Gateway — a separate step covered below.

Finding your UTR

The UTR appears on:

• The letter HMRC sends after registration (form SA250)
• Previous Self Assessment letters and statements
• Some HMRC online services once the account is linked

If the UTR is lost, taxpayers can sign into their personal tax account or contact HMRC. HMRC will not give a UTR over the phone without identity checks.

The UTR is different from a National Insurance number. Both may appear on correspondence, but only the UTR is used to file Self Assessment.

Setting up a Government Gateway account

Online filing runs through HMRC's digital services, which require a Government Gateway user ID and password.

New filers receive an activation code by post after registering. The code must be used within 28 days. Once activated, the account links to the UTR and opens access to file returns, view tax calculations, and manage payments.

Existing Government Gateway accounts can often be reused if they are already linked to the correct UTR. Taxpayers who have filed before but forgot login details can recover them through GOV.UK's account recovery process.

Two-step verification is part of the sign-in process. HMRC may send a code by text or through an authenticator app depending on how the account is set up.

Full sign-in instructions are on Log in and file your tax return online.

Gathering information before filing

A complete return needs accurate figures for the full tax year. Starting early reduces errors and makes the January deadline less stressful.

Typical records include:

• Bank statements showing business and personal income
• Invoices issued and received
• Receipts for allowable expenses
• P60, P45, or P11D forms from employers (if any PAYE work happened alongside self-employment)
• Records of rental income and costs
• Dividend vouchers from companies
• Details of pension contributions and Gift Aid donations (these can affect the tax bill)
• Student loan plan type, if repayments apply

Self-employed filers report business income and expenses on the return. Sole traders with straightforward affairs may use the short pages; more complex cases use the full self-employment pages. The sole trader Self Assessment guide covers records and expenses in more detail.

Filling in the return online

HMRC's online service steps through the return section by section. Taxpayers answer questions about income types, and the system opens only the pages that apply.

The main stages are:

1. Confirm personal details — name, address, and National Insurance number
2. Declare income sources — employment, self-employment, property, dividends, interest, and others
3. Complete each income section — enter totals for the tax year, not monthly figures
4. Tax reliefs and allowances — pension payments, charitable giving, marriage allowance transfers
5. Review the tax calculation — HMRC shows an estimated bill before submission
6. Submit and receive confirmation — a reference number proves the return was filed on time

The online system calculates tax automatically and flags obvious errors, such as figures that do not match records HMRC already holds.

Paper returns (form SA100) are still available but have an earlier deadline of 31 October after the tax year ends. Most filers use the online service because the deadline is later and calculations are automatic.

Deadlines that matter

Missing a deadline has real costs. The key dates for the tax year ending 5 April are:

What	Online filing	Paper filing
Register (if newly self-employed)	5 October after the tax year	5 October after the tax year
Submit the return	31 January following the tax year	31 October following the tax year
Pay the tax owed	31 January following the tax year	31 January following the tax year

So for the 2025–26 tax year (6 April 2025 to 5 April 2026), the online filing and payment deadline is 31 January 2027.

Taxpayers who owe enough tax may also need payments on account — advance payments towards the next year's bill, due on 31 January and 31 July. The payments on account guide explains when these apply and how to budget for them.

The Self Assessment deadlines guide lists every date in one place.

Paying the tax bill

After filing, HMRC issues a tax calculation. Payment is due by the same 31 January deadline unless a Time to Pay arrangement is agreed.

Ways to pay include:

• Online banking — using HMRC's bank details and a payment reference
• Debit or corporate credit card through HMRC's online service (card fees may apply)
• Direct Debit — including a budget payment plan to spread costs
• PAYE coding adjustment — for smaller amounts, sometimes collected through a tax code (if employed as well)

Bank transfers can take several working days. Paying a few days before 31 January avoids late-payment interest.

Full payment options are listed on Pay your Self Assessment tax bill.

After filing

Once submitted, the return can still be amended if a mistake is found. Online amendments are possible through the same Government Gateway account. Paper filers use form SA100 with the corrected pages.

HMRC may open an enquiry into a return if figures look unusual or records are incomplete. Keeping clear records for at least five years after the filing deadline supports any later questions.

If no return is needed in future years, taxpayers can contact HMRC to ask for the Self Assessment requirement to be removed — but only if circumstances have genuinely changed.

Common problems

No UTR yet — filing cannot proceed until the number arrives. Register early to allow time for post.

Lost Government Gateway access — use GOV.UK account recovery or call HMRC's Self Assessment helpline.

Return filed but payment missed — the return itself may be on time, but unpaid tax accrues interest from 1 February. Penalties for late payment follow separate rules.

Income from employment and self-employment — both sections of the return need completing. PAYE tax already paid is credited against the final bill.

Company directors — a company return does not replace a personal return when untaxed income exists. The director Self Assessment guide covers when directors need to file.

Your next step

Taxpayers who know they need to file can register for Self Assessment at GOV.UK if they have not already — that starts the process of receiving a UTR and activating online filing: Register for Self Assessment.