The online Self Assessment return for 2025/26, and any tax owed, are due by midnight on 31 January 2027. A late return normally means an immediate £100 penalty.
If your Self Assessment bill is £1,000 or more, and less than 80% of your tax was already collected at source (e.g. through PAYE), HMRC asks for two advance payments towards next year: 31 January and 31 July, each half of this year's bill.
In your first year you may owe the full bill plus a 50% payment on account on the same 31 January. A £2,000 bill can mean £3,000 due in January.
Set money aside monthly. If you know next year will be lower, you can ask HMRC to reduce payments on account.
Work out your own figure: our UK Side-Hustle Tax Set-Aside 2026/27 spreadsheet (free lite version, or the full version for £4 on Payhip) calculates the extra tax and Class 4 NI on top of your PAYE salary.
Written with the help of an AI assistant and checked against GOV.UK on 21 Oct 2026. General information, not tax advice; if in doubt, contact HMRC or a qualified adviser.