Do I need Self Assessment?

The HMRC self-assessment criteria

HMRC requires you to file a Self Assessment tax return if any of the following are true for the tax year. The list below covers the common triggers for individuals; partnerships and trustees have additional rules.

• You were self-employed as a sole trader and earned more than £1,000 in trading income (the trading allowance threshold).
• You were a partner in a business partnership.
• You earned untaxed income over £2,500 — for example tips, commission, or freelance side work that wasn't taxed at source.
• Your property income exceeded £1,000 (the property allowance) before expenses, or £2,500 after expenses if higher.
• You received dividends over £10,000 in the year.
• You had to pay the High Income Child Benefit Charge (income over £60,000 with Child Benefit being claimed).
• Your income from savings and investments was £10,000 or more.
• You sold assets and the capital gain exceeded the annual exempt amount, or the disposal proceeds exceeded four times the annual exempt amount.
• Your total income was over £150,000.
• You needed to claim income tax relief on pension contributions outside of PAYE, or on charitable gifts, or on EIS/SEIS investments.

Side hustles and the £1,000 trading allowance

The £1,000 trading allowance is a "no-need-to-file" threshold for casual self-employment income. If you earned, say, £800 from selling crafts on Etsy, and that was your only untaxed income, you neither need to register nor file. If you earned £1,500 from the same activity, you must register and file — although you can either claim the £1,000 trading allowance against the gross or claim actual expenses, whichever is more generous.

The trading allowance is a pre-tax exemption, not a personal allowance. Use the side hustle self-assessment checker for a first pass on whether your activity crosses the line.

Deadlines and penalties

The 2026/27 tax year ends on 5 April 2027. Paper returns must be filed by 31 October 2027; online returns by 31 January 2028. The same 31 January deadline applies to paying any tax owed, and to the first payment on account for 2027/28 if your prior-year liability was £1,000+.

Missing the filing deadline triggers an immediate £100 penalty even if no tax is owed. Continuing late filing escalates: daily penalties of £10/day after three months, then 5% of tax due (or £300, whichever is higher) at six and twelve months. Late payment penalties are separate and start at 5% of unpaid tax after 30 days.

Related on this site

Run the Self Assessment readiness checklist a few weeks before the filing deadline, and use the side hustle checker to test whether casual income crosses the trading-allowance threshold. The January 2027 self-assessment checklist walks through the actual filing steps for the current cycle.

Common reasons HMRC may expect a return

• Self-employed income, side-hustle profit or taxable platform income above the simple exemptions.
• Rental income, significant untaxed investment income, or capital gains outside wrappers.
• Other specific HMRC triggers that payroll alone does not settle.

Common reasons people file when they may not need to

People often assume any side income means automatic Self Assessment. In reality, some income is covered by allowances or has a clearer “tell HMRC another way” route instead. That is why HMRC's own checker matters.

Best next page

Use Side Hustle and Self Assessment Checker first, then use the official HMRC “check if you need a tax return” tool for the formal answer.

The 5 October registration deadline

The deadline most first-timers miss is not the January filing date — it is the registration date. If you have a new reason to file for the first time, you must tell HMRC by 5 October following the end of the tax year in which the liability arose. So for income earned in the 2026/27 tax year (which ends on 5 April 2027), you must register by 5 October 2027. Registering is what generates your Unique Taxpayer Reference (UTR) and, if you are newly self-employed, your Class 2/Class 4 National Insurance record.

Registration is not instant. HMRC posts an activation code for the online service, which can take up to ten working days to arrive (longer if you are abroad). Leaving registration until December — when the system is busiest — is the single most common reason people end up filing late through no real fault of their own. If you know a return is coming, register as soon as the tax year ends rather than waiting for the autumn deadline. A late-notification failure can itself attract a penalty based on the “potential lost revenue” where tax was due, so it is not a free deadline to miss.

Payments on account: the cash-flow shock

Many first-time filers are blindsided not by the tax itself but by payments on account. If your Self Assessment liability for a year is £1,000 or more, and less than 80% of your tax was collected at source (for example through PAYE), HMRC asks you to pay towards next year’s bill in two instalments. Each instalment is 50% of the current year’s liability.

That means your first January bill can be 150% of the tax you actually owe: the balancing payment for the year just ended, plus the first 50% payment on account for the year ahead. The second payment on account then falls due on 31 July. A worked illustration: if your 2026/27 liability is £4,000, then by 31 January 2028 you pay the £4,000 balance plus a £2,000 payment on account, and a further £2,000 on 31 July 2028 — £8,000 of cash out in your first cycle even though the tax year only generated £4,000. If you expect your income to fall, you can apply to reduce your payments on account, but reducing them too far triggers interest on the shortfall.

How to stop filing (deregistering)

Self Assessment does not switch itself off. If your circumstances change — you stop trading, sell the rental property, drop below the relevant thresholds, or your only income is now taxed under PAYE — you must actively tell HMRC, or returns will keep being issued and the £100 late-filing penalty will keep applying even when no tax is due.

To come out of Self Assessment you tell HMRC you no longer meet any of the criteria (online through your account, or by phone or post). If you have ceased self-employment you give the date the business stopped; HMRC closes the Class 2 NI record from that date. You will normally still have to file a final return covering the part-year up to the date you stopped. Until you receive written confirmation that the requirement has ended, keep filing — a verbal assurance is not enough to cancel the obligation.

Quick checklist: who must file

As a one-glance summary, HMRC will normally expect a 2026/27 return if any of the following applied during the year:

• Self-employed trading income above the £1,000 trading allowance, or you were a partner in a partnership.
• Rental or other property income above the £1,000 property allowance.
• Total income above £150,000.
• You owed the High Income Child Benefit Charge (income over £60,000 with Child Benefit in the household).
• Untaxed income — tips, commission, freelance work — that PAYE did not capture.
• Capital gains above the annual exempt amount, or disposal proceeds well above it, on assets held outside an ISA or pension.
• Significant savings or dividend income above the relevant allowances.
• You need to claim relief that PAYE cannot give automatically — for example higher-rate pension relief or EIS/SEIS/VCT relief.
• Foreign income, or you are non-UK-resident with UK income that needs reporting.

If none of these apply and all your tax is settled through your tax code, you almost certainly do not need to file — and if HMRC has issued a notice anyway, ask them to withdraw it.