Self Assessment tax returns 2026: deadlines, penalties and common mistakes

A Self Assessment tax return is the route HMRC uses to collect tax where income has not been fully taxed at source. HMRC says a return may be required for self-employment, partnership income, capital gains, the High Income Child Benefit Charge and other circumstances, and a return must be sent if HMRC asks for one. HMRC guidance on who must send a return.

Who should check their filing position?

Typical cases include sole traders, landlords, partners, individuals with taxable gains, directors with dividends or benefits, and employees with material untaxed income. The risk is not only whether tax is due, but whether HMRC expects a return.

Self Assessment tax return deadlines

For the 2025/26 tax year, HMRC states that paper returns must be received by 31 October 2026 and online returns by 31 January 2027. Where tax is to be collected through a tax code, the online return deadline is 30 December 2026. HMRC Self Assessment deadlines.

Penalties and payments

HMRC’s late filing penalties start with an initial £100 charge, with further daily and percentage-based penalties after three, six and twelve months. HMRC Self Assessment penalties. The balancing payment is generally due on 31 January and payments on account are normally due on 31 January and 31 July. HMRC payment guidance.

Common mistakes

• Starting the return in January rather than preparing records throughout the year.
• Omitting bank interest, dividends, benefits or rental income.
• Claiming expenses without evidence.
• Reducing payments on account without a reliable profit forecast.
• Ignoring a notice to file because no tax is expected.