The deadline for submitting an online Self-Assessment and paying tax you owe is midnight on the 31st of January following the end of the tax year. If it is not filed on time you will face penalties.
Anyone who misses the Self-Assessment filing deadline will receive a £100 fine, this is imposed immediately whether you owe tax or not. If the tax return is more than 3 months late HMRC can then impose an extra daily fine of £10 for the next 90 days if you fail to file your return during this time, potentially increasing the total penalty by £900 to £1000.
Penalties for late filing are also imposed after 6 months and then 12 months, which are based on 5% of the amount of tax you owe or £300 (whichever is greater).
The penalties for being 6 months late and 12 months late are subject to the interaction rule and taken together they should not exceed 100% the tax due.
On top of this you will be charged penalties and interest on any unpaid tax. Additional penalties may also apply if HMRC believe you deliberately have withheld information and a return is late.