Self Assessment involves completing an online or paper tax return. There can be several reasons to why HMRC may need a person to complete a tax return - for example if you're self-employed, a company director, a trustee or if you have foreign income.
You need to register for Self Assessment before you can get a tax return. HMRC will send you a Unique Taxpayer Reference by using the information you provide and will send you a letter each year, usually in April, telling you to complete your tax return. If you received your Unique Taxpayer Reference but have not received a letter telling you to complete a tax return by the end of April, you should contact your Tax Office.
The deadline for paper tax return is 31 October. If you want to send your tax return online, you will be given an extra 3 months and the deadline will be extended until January 31. You'll be charged a £100 fine and can expect to be charged interest if your tax return isn't received on time.