Self Assessment Tax Return: A Complete Guide for 2025/26

Tax Guides · 9 min read · Updated 2026-03-20

Learn who needs to file a self assessment tax return, key deadlines, what income to report, and how to avoid penalties from HMRC.

Written by Krishna, Founder & HMRC-registered tax agent at TaxRoot.

What happens if I miss the 31 January deadline?

If you miss the 31 January online filing deadline, HMRC will charge an automatic £100 penalty, even if you do not owe any tax. If your return is more than 3 months late, you will face additional daily penalties of £10 per day for up to 90 days. Further penalties…

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