How to file a Self Assessment tax return

You generally need to file if, in the tax year, you were self-employed as a sole trader with turnover above the trading allowance, a partner in a business partnership, or a company director with untaxed income.

You also need to file if you had untaxed income that HMRC cannot collect through your tax code: rental income above the property allowance, dividends above the dividend allowance, savings interest above the personal savings allowance, foreign income, or capital gains above the annual exempt amount.

Two threshold-based traps catch people who thought they were purely employed. The High Income Child Benefit Charge applies where you or your partner receive Child Benefit and one of you has income above a threshold. And if HMRC has sent you a notice to file, you must file — even if you believe you owe nothing — unless HMRC formally withdraws it.

GOV.UK has a free checker that answers the question in a couple of minutes. Use it rather than guessing, because a wrong guess produces an automatic penalty.

If this is your first Self Assessment return, you must register with HMRC by 5 October following the end of the tax year in question. So for the year ending 5 April 2026, registration is due by 5 October 2026 and the return by 31 January 2027.

Registration produces a Unique Taxpayer Reference, posted to you, and then an activation code for the online service — also posted. That two-step postal process takes time, and it is why leaving registration until January is a genuine problem rather than an inconvenience.

Failing to notify HMRC that you are chargeable can attract a penalty based on the tax due, separate from the late filing penalty.

If you have filed before, you keep the same Unique Taxpayer Reference for life and simply sign in.

Sign in to your HMRC online account and work through the return. It asks about each type of income, then about expenses, reliefs and allowances, and produces a calculation as you go.

Have ready: records of income and expenses, P60 and P45 forms, P11D for benefits in kind, bank interest statements, dividend vouchers, pension contribution records, Gift Aid donations, and any records relating to property or capital gains.

Claim what you are entitled to. Allowable business expenses must be wholly and exclusively for the business, which is stricter than it sounds — a phone used for both work and personal use is apportioned, not claimed in full. The trading allowance and the property allowance let you receive a small amount of income free of tax and without records, which suits people with a modest side income.

Pension contributions and Gift Aid donations extend your basic rate band, which is how higher-rate taxpayers claim the additional relief they are owed. This is one of the most commonly missed claims on the whole return.

Submit before 31 January and pay at the same time. Keep the submission receipt.

If your Self Assessment bill exceeds a threshold and less than 80 per cent of your tax is collected at source, HMRC requires payments on account: two advance instalments toward next year's bill, due on 31 January and 31 July, each half of the previous year's liability.

This is the reason a first Self Assessment bill often feels like one and a half times what was expected — you are paying last year's tax plus the first instalment of next year's at the same time. If you know your income has fallen, you can apply to reduce payments on account, though HMRC charges interest if you reduce them too far.

Penalties escalate on a fixed schedule. A £100 fixed penalty applies immediately the deadline passes, even if no tax is owed. After three months, daily penalties accrue. After six and twelve months, further penalties are charged based on the tax due. Late payment attracts separate penalties and interest.

Penalties can be appealed on the basis of a reasonable excuse — serious illness, a bereavement, a genuine service failure by HMRC. Being busy, finding the system difficult, or relying on someone else to file are not reasonable excuses.

If you cannot pay, contact HMRC before the deadline. Time to Pay arrangements are routinely agreed for people who ask early, and considerably harder to obtain once enforcement has begun.