When is the UK self-assessment deadline?

The online filing deadline is midnight on 31 January 2027 for the 2026/27 tax year. Paper returns are due by 31 October 2026. Late filing triggers an automatic GBP 100 penalty even if no tax is owed.

When are payments on account due?

If your self-assessment bill is GBP 1,000 or more and less than 80 percent of tax was collected at source, HMRC asks for two payments on account. The first is due 31 January, the second 31 July, each at 50 percent of the prior year bill.

What happens if I file late?

You get an automatic GBP 100 penalty the day after the deadline. After three months HMRC adds GBP 10 a day for up to 90 days. Tax paid late accrues interest at the HMRC rate (base rate plus 2.5 percent).

Which HMRC supplementary pages do I need?

Employees use SA102. Sole traders use SA103S (short) or SA103F (full) depending on turnover. Landlords file SA105. Capital gains use SA108. Foreign income uses SA106. The PaySlipIQ helper picks the right pages from your inputs.

UK Self-Assessment Helper

Estimate your 2026/27 self-assessment tax bill, see whether payments on account apply, and get the deadline, penalty schedule, allowable expenses checklist and the HMRC supplementary pages you need to file.

Employment income (PAYE, annual)DividendsRental income (annual)Sole-trader profit (annual)Capital gains (annual)Pension contributions (gross)Charitable donations (Gift Aid)Student loan planNational Insurance classMy spouse or civil partner earns under the personal allowance (Marriage Allowance)