Self Assessment
UK tax glossary · Last reviewed: April 2026
Self-employed people, landlords, and those with significant untaxed income file an SA100 return after the tax year ends. The main online deadline is 31 January (for the tax year ending the previous April).
You must also register for Self Assessment if: you had income over £100,000; you received income from savings, investments or dividends above allowances; or you are a company director. Registration deadline for new self-employed people is 5 October after the tax year.
Late filing triggers an automatic £100 penalty, rising to £10/day after 3 months. Interest also accrues on unpaid tax.
Worked example
Self-employed profit: £30,000 (2026/27). Income Tax: (£30,000 − £12,570) × 20% = £3,486. Class 4 NI: (£30,000 − £12,570) × 6% = £1,045.80. Class 2 NI: abolished from April 2024. Total SA bill due 31 January 2028: ~£4,532 (plus first payment on account of ~£2,266).
Common questions
Do I need to register for Self Assessment if I am employed but have a side hustle?
Yes, if your side-income exceeds £1,000 (the trading allowance) you must register by 5 October after the tax year and declare it on a return.
What is the difference between a balancing payment and a payment on account?
A balancing payment settles any shortfall from the previous year. Payments on account are advance payments toward the current year's bill — each equal to half the previous year's liability, due 31 January and 31 July.
Related resources
TaxHelper provides general information based on published HMRC rates and guidance. It is not regulated financial or tax advice. For decisions involving significant sums, complex circumstances, or if you are unsure, speak to a qualified accountant or HMRC directly.