You must file a Self Assessment tax return with HMRC every year if you are self employed (and earn more than £1,000) or you are a partner in a business partnership. You need to file a return regardless of whether you have any tax or National Insurance to pay.
In addition, if your main income is from employment, but you earn money form renting our property, earn additional money in tips and commission that isn’t taxed through payroll, you have income from savings and investments exceeding £10k, if you earn dividends or foreign income. You also need to submit a tax return if you earn over £100k per year, if you receive Child Benefit and you or your partner earn over £50k per year.
If you are a higher rate taxpayer, you may wish to file a self assessment to claim tax relief on Gift Aid donations and personal pension payments.
If you have Capital Gain Tax to pay, and you have used the quick service, you will also need to declare this in a Self Assessment.