Self-assessment

A self-assessment tax return is how HMRC collects income tax from individuals whose tax is not automatically deducted through PAYE. This typically applies to:

• Self-employed people and sole traders.
• Partners in a business partnership.
• Anyone with additional income, such as from investments or rental property.

You will normally also need to file if you:

• Have untaxed income above the reporting threshold — for example dividends, savings interest or rent.
• Need to claim higher-rate or additional-rate tax relief on personal pension contributions.
• Are liable to the High Income Child Benefit Charge.
• Have capital gains to report, or want to claim losses to carry forward.

The income thresholds that trigger a return change from time to time, so check the current figures on GOV.UK or with your accountant rather than relying on last year’s rules.

See your income tax position

Our free UK tax calculator shows your income tax and National Insurance across the bands, so you can see the effect of a pension contribution before you make it.

Missing a self-assessment deadline results in penalties. The critical dates are:

• 5 October — register for self-assessment, if it is your first time.
• 31 October — deadline for paper tax returns.
• 31 January — deadline for online returns, and for paying any tax owed for the previous tax year.
• 31 July — second payment on account, where one applies.

If a deadline falls on a weekend or bank holiday, make sure your return and payment reach HMRC by the preceding working day rather than assuming an extension.

• Your Unique Taxpayer Reference (UTR) and National Insurance number.
• P60 and P45 forms, where applicable.
• Details of all income — employment, dividends, rental and any foreign income.
• Records of allowable expenses, such as office costs, travel and professional fees.
• Bank interest statements and investment income details.
• Pension contribution statements, including any employer contributions, so allowances are calculated correctly.
• Gift Aid donations, which extend your basic-rate band.

1. Log in to HMRC online services

Sign in with your Government Gateway ID, go to the self-assessment section and start your return for the relevant tax year.

2. Complete the relevant sections

Depending on your income sources you may need additional pages — self-employment, property, capital gains or foreign income. Check every figure against your records rather than working from memory.

3. Claim your allowable expenses

Make sure you deduct everything you are entitled to, which may include:

• Office costs such as stationery and software.
• Travel expenses — fuel, parking and public transport.
• A proportion of the costs of running a home office.
• Professional fees, subscriptions and training directly related to your work.

4. Check and submit

Review your entries thoroughly, submit, and save a copy of the return and the calculation. Keep your records for at least the period HMRC can open an enquiry.

Missing the deadline. A late return triggers an automatic fixed penalty, with further penalties and interest accruing the longer it goes unfiled — even if no tax is owed.

Inaccurate figures. Errors can lead to penalties or an HMRC enquiry. Careless mistakes are treated more seriously than the tax at stake often warrants.

Forgetting to claim expenses or pension relief. Higher and additional-rate taxpayers must claim the extra relief on personal pension contributions through their return. It is one of the most commonly missed reclaims in the UK.

Ignoring payments on account. Where your liability exceeds the threshold, HMRC requires advance payments toward the following year. The first January bill can therefore be considerably larger than expected.

An accountant files the return; a financial adviser reduces what it says. The planning that changes your bill happens before the tax year ends, not in January. An adviser can:

• Use pension contributions and carry-forward to manage your marginal rate, including recovering the personal allowance.
• Check whether the tapered annual allowance applies to you before you over-contribute.
• Plan the timing of dividends, bonuses and capital gains across tax years.
• Structure investments across ISAs and pensions so future returns generate less reportable income.