UK Self Assessment Step by Step 2026/27

Who Must File a Self Assessment Return

Most employees have all their tax collected automatically through PAYE and never touch Self Assessment. You are pulled into the system when you have income that HMRC cannot tax at source, or when special charges apply. For 2026/27 the common triggers are:

• Self-employment with gross income over £1,000 (the trading allowance).
• Being a partner in a business partnership.
• Untaxed income — rental profit, dividends above the £500 allowance, savings interest beyond the Personal Savings Allowance, tips or foreign income.
• Total income over £150,000.
• Liability for the High Income Child Benefit Charge.
• Capital gains above the annual exempt amount, or where you must report a property disposal.

If none of these apply you generally do not need to file — but if HMRC has sent you a notice to file, you must complete a return (or ask for it to be withdrawn) even if no tax is due. Use the income tax calculator to estimate your overall liability first.

Registering and Getting Your UTR

Before you can file you must register with HMRC and receive a Unique Taxpayer Reference (UTR) — a 10-digit number that identifies you in the Self Assessment system. The deadline to register is 5 October following the end of the tax yearin which the liability first arose. For income earned in 2025/26, that means registering by 5 October 2026.

Registration routes differ slightly: newly self-employed people register for Self Assessment and Class 2/Class 4 National Insurance together; those with untaxed income but no self-employment use a separate form. HMRC then posts your UTR, followed by an activation code for your online Government Gateway account. Allow two to three weeks for the whole process — leaving it late risks a “failure to notify” penalty based on the tax you owe.

Self-employed taxpayers can estimate their bill, including Class 4 NI, with the self-employed tax calculator before they register, so the eventual liability is no surprise.

Key Dates

The Self Assessment calendar has a small number of hard deadlines. Missing any of them triggers automatic penalties, so they are worth committing to memory:

The vast majority of taxpayers file online, gaining three extra months over the paper deadline. There is one extra wrinkle: if you want HMRC to collect a tax bill under £3,000 through your PAYE code rather than as a lump sum, you must file online by 30 December.

Payments on Account

Payments on account catch out almost every first-time filer. They are advance payments towards next year's tax, required when your Self Assessment liability exceeds £1,000 and less than 80% of your tax was collected at source.

Each payment on account is 50% of the prior year's liability, due on 31 January and 31 July. The sting is in your first year: the January payment combines the balancing payment for the year just ended plusthe first 50% payment on account — effectively 150% of a year's tax in one go. Budgeting for this in advance, ideally by setting aside tax as you earn, avoids a nasty cash-flow shock.

If your income has fallen, you can apply to reduce your payments on account — but under-pay and HMRC charges interest on the shortfall, so reduce them only if you are confident.

What to Gather Before You Start

Pulling everything together first makes the return far quicker. Have to hand:

• Your UTR and Government Gateway login.
• P60 and/or P45 for employment income; any P11D for benefits in kind.
• Bank and building society statements showing untaxed interest.
• Dividend vouchers and investment statements.
• Rental income and allowable expense records (for the SA105 pages).
• Receipts and totals for self-employed business expenses.
• Records of personal pension contributions and Gift Aid donations.
• Details of any capital gains or losses.
• Last year's return, for reference and to check payments on account already made.

Keep these records for at least five years after the 31 January filing deadline — HMRC can open an enquiry well after you file.

Common Boxes and Supplementary Pages

The online return tailors itself to your circumstances, adding supplementary pages as you answer the opening questions. The pages most people encounter are:

• Employment (SA102): pay and tax from your P60/P45, plus benefits from your P11D.
• Self-employment (SA103): turnover, allowable expenses and the resulting profit, which feeds Class 4 NI.
• Property (SA105): rental income, allowable expenses and the residential finance-cost (mortgage interest) restriction.
• Capital gains (SA108): disposals, gains, losses and reliefs.
• Additional information (SA101): other reliefs, including pension contributions and Gift Aid that extend your basic-rate band.

Take particular care entering pension contributions and Gift Aid — they reduce your tax and, for those near £60,000, can remove the High Income Child Benefit Charge or restore part of the Personal Allowance lost in the £100,000–£125,140 band.

Allowable Expenses

For the self-employed, expenses must be incurred “wholly and exclusively” for the business. Commonly claimed costs include office and premises costs, stock and materials, business travel and mileage, a proportion of home running costs (or the simplified flat rate), professional fees and subscriptions, marketing, and equipment (through capital allowances or the Annual Investment Allowance).

Employees can claim a narrower set — approved professional subscriptions and the £6/week working-from-home allowance where eligible. Landlords claim allowable property expenses on the SA105 pages, though residential mortgage interest is restricted to a 20% tax credit rather than a full deduction.

If your self-employed turnover is modest, the £1,000 trading allowance may beat itemising expenses; you can claim whichever is higher, but not both.

Penalties for Filing or Paying Late

HMRC's late-filing penalties escalate quickly and apply even if you owe no tax:

Late payment carries separate penalties: 5% of the unpaid tax at 30 days, again at six months and again at twelve months, plus daily interest on the outstanding balance. The two sets of penalties stack, so an unfiled, unpaid return can cost well over £1,000 on top of the tax itself.

Time to Pay if You Cannot Pay

If you cannot pay your bill in full, do not bury your head in the sand. HMRC offers Time to Pay arrangements that spread the liability over monthly instalments, typically up to 12 months.

If you owe less than £30,000, have filed your return and have no other outstanding tax, you can usually set up a Time to Pay plan online without phoning HMRC. Interest still accrues on the deferred amount, but arranging a plan before the deadline avoids the worst of the late-payment penalties — far cheaper than simply missing the date.

Making Tax Digital for Income Tax

The biggest change on the horizon is Making Tax Digital for Income Tax Self Assessment (MTD ITSA), which replaces the single annual return with digital record-keeping and quarterly updates for affected taxpayers.

• April 2026: mandatory for sole traders and landlords with qualifying gross income above £50,000.
• April 2027: threshold drops to £30,000.
• April 2028: a £20,000 threshold has been announced.

Under MTD ITSA you must keep digital records and submit quarterly updates through compatible software, then a final declaration after the tax year. If your income is below the relevant threshold you continue with the normal annual return for now — but moving to bookkeeping software early makes the eventual transition painless. See the MTD ITSA explained guide for the full detail.

The Five Steps at a Glance

1. Check whether you need to file. Confirm you meet a Self Assessment trigger — self-employment income over £1,000, untaxed income such as rent or dividends, total income over £150,000, the High Income Child Benefit Charge, or capital gains above the annual exempt amount.
2. Register and get your UTR. Register with HMRC by 5 October after the end of the tax year. HMRC issues a 10-digit Unique Taxpayer Reference (UTR) and an activation code for your online account — allow up to two to three weeks.
3. Gather your records. Collect your income figures, P60/P45, P11D, bank and dividend statements, rental records, expense receipts, pension and Gift Aid contributions and your prior-year return before you start.
4. Complete and file the return online by 31 January. Work through the relevant pages (employment, self-employment, property, capital gains), claim allowable expenses and reliefs, check the calculation and submit before 11.59pm on 31 January.
5. Pay your tax — and any payments on account. Pay the balancing payment by 31 January, plus the first payment on account if your bill exceeds £1,000 and under 80% is taxed at source. The second payment on account is due 31 July.

Official References

• gov.uk: Self Assessment tax returns
• gov.uk: Self Assessment deadlines
• gov.uk: Payments on account
• gov.uk: If you cannot pay your tax bill on time (Time to Pay)
• gov.uk: Making Tax Digital for Income Tax