Self-Assessment

Self-Assessment is HMRC’s annual tax return system for taxpayers whose liability isn’t fully settled through PAYE. The default audience is the self-employed and landlords, but it also catches: higher earners above £100,000 (because of the personal allowance taper), anyone receiving untaxed savings interest above the personal savings allowance, anyone receiving dividends above the dividend allowance, partners in partnerships, ministers of religion, and trustees.

The deadlines that matter:

• 31 October: paper return for the tax year that ended the previous 5 April.
• 31 January: online return + first balancing payment + first payment on account.
• 31 July: second payment on account.

The payments on account system surprises a lot of first-time self-assessment filers. If the prior-year tax bill was over £1,000 and less than 80% of total tax was deducted at source, HMRC requires two advance payments of 50% each toward the next year’s liability — meaning a first-time filer in January can owe 150% of the year’s tax in one go.

Use the self-assessment calculator to model the balancing payment plus payments on account, or the National Insurance calculator for the Class 4 element.