Self-Assessment Tax Returns

HMRC requires a Self Assessment tax return from anyone who falls into one or more of the following categories in a given tax year. If you are in doubt, it is safer to register and file than to assume you do not need to — failing to file when required carries heavier penalties than filing unnecessarily.

• Self-employed sole traders whose gross income from self-employment exceeded £1,000 in the tax year (the trading allowance threshold), even if expenses reduced their profit to zero
• Partners in a business partnership — each partner must file an individual return, and a nominated partner must also file the partnership return (SA800)
• Company directors who receive any income — salary, dividends, benefits in kind, or loans — from the company, unless it is a non-profit organisation and they received nothing
• Landlords with rental income above £1,000 — the property allowance applies below this threshold but a return is needed once it is exceeded
• Anyone with untaxed income from savings interest, investments, or dividends above HMRC's relevant thresholds
• Anyone who earned over £100,000 in the tax year — the personal allowance tapers above this figure and PAYE cannot account for it automatically
• Those subject to the High Income Child Benefit Charge (HICBC) — if you or your partner received Child Benefit and either of you earned over £60,000, the charge must be declared and paid via Self Assessment
• Anyone with capital gains above the annual exempt amount (£3,000 from 2025/26) or foreign income requiring declaration

HMRC may send you a Notice to File even if you do not think you need to file. Receiving one creates a legal obligation — you must file by the deadline or face penalties, even if your tax liability turns out to be nil. If you believe you have been sent one in error, contact HMRC to confirm before the deadline, do not simply ignore it.

The UK tax year runs from 6 April to 5 April. Several deadlines follow the end of each tax year, and missing them triggers automatic penalties regardless of whether you owe any tax.

• 5 October — deadline to register for Self Assessment with HMRC if you became liable in the previous tax year. For the self-employed, this is done using the CWF1 form or the online equivalent, which also registers you for Class 2 and Class 4 National Insurance. Others use form SA1. HMRC will then issue your Unique Taxpayer Reference (UTR) number by post within ten working days.
• 31 October — deadline for submitting a paper Self Assessment return. Filing on paper is increasingly rare and agents sometimes use it for complex returns, but most taxpayers file online.
• 31 January — deadline for submitting your online return and paying the balancing payment (any remaining tax owed for the previous year after payments on account). This is also the due date for the first payment on account for the current tax year (see below).
• 31 July — deadline for the second payment on account for the current tax year.

HMRC sends reminders but is not legally required to do so — it is your responsibility to know and meet the deadlines. If you use an accountant, ensure they have all your records well before the January deadline. Accountants become extremely busy in November and January and many set their own earlier deadlines for client submissions.

Payments on account are one of the most misunderstood features of the Self Assessment system. They are advance payments towards your next year's tax bill, calculated on the assumption that your income this year will be similar to last year's.

The system applies if your previous year's Self Assessment tax bill (excluding Capital Gains Tax and student loan repayments) exceeded £1,000. In that case, HMRC requires you to make two advance payments:

• First payment on account: Equal to 50% of your previous year's bill, due on 31 January alongside the balancing payment for the previous year
• Second payment on account: Also equal to 50% of your previous year's bill, due on 31 July

The practical effect in your first year of Self Assessment can be dramatic. Suppose your first-year tax liability is £4,000. On 31 January you owe: £4,000 (balancing payment) plus £2,000 (first payment on account for the following year) — a total of £6,000. The second payment on account of £2,000 then falls due six months later in July. Many first-time filers are unprepared for this and struggle with the cash-flow impact. Putting aside at least 30% of all income from the moment you start trading is the most reliable way to avoid a crisis.

If your income has fallen and you expect your tax liability to be lower than the previous year's, you can apply to reduce your payments on account online via your HMRC account or by submitting form SA303. This is a legitimate cash-flow tool — HMRC will not refuse a reasonable reduction request. However, if you reduce them and your actual liability turns out to be higher than the reduced amount, HMRC will charge interest on the shortfall from the original due dates. The 5% surcharge does not apply (that is for late payment of the balancing payment), but interest can still mount up over several months.

When you file your return, HMRC calculates any balancing payment — the difference between what you actually owe and what you have already paid through payments on account. If you overpaid, HMRC will refund the difference or credit it against future payments. Making Tax Digital for Income Tax (MTD for ITSA) is due to change how this works for some taxpayers from April 2026 onwards, with quarterly reporting requirements.

HMRC's penalty regime for Self Assessment is automatic and escalates sharply the longer a return or payment is overdue. Penalties apply even if you owe no tax.

Late filing penalties:

• Immediate £100 penalty if the return is filed even one day after the 31 January deadline
• After 3 months: daily penalties of £10 per day, continuing for up to 90 days (maximum £900)
• After 6 months: an additional penalty of £300 or 5% of the tax due, whichever is greater
• After 12 months: a further penalty of £300 or 5% of the tax due, with higher percentages (up to 100% of the tax) if HMRC considers information was deliberately withheld

Late payment penalties and interest:

• Interest accrues from the day after the payment deadline at HMRC's current late payment rate (linked to the Bank of England base rate plus 2.5%)
• After 30 days: a 5% surcharge on the unpaid amount
• After 6 months: a further 5% surcharge
• After 12 months: another 5% surcharge

If you cannot pay on time, contact HMRC before the deadline to request a Time to Pay arrangement. HMRC will generally agree to monthly instalments if you approach them proactively. Interest continues to accrue under a Time to Pay plan, but the 5% surcharge penalties can be avoided if an arrangement is in place before the surcharge dates fall. Ignorance of the deadline, pressure of work, or reliance on an accountant who fails to file on time are not accepted as reasonable excuses for late penalties.